DANNY WALLACE AND I MEET IN A pub in London's Islington. A gastropub with topiary in the beer garden and organic parma ham on the tapas menu.

Wallace looks around him with an air of disbelief. One day he was in the East End, slugging pints with his mates in a dingy old boozer, the next he was Islington Man, married with a mortgage, scatter cushions and a book on DIY. He casts a hand around him at our surroundings and asks: "How did this happen?"

At the same time Wallace, the writer, TV presenter and lad-about-town, was staring his 30th birthday in the face and wondering what happened to his youth, he discovered a childhood address book – with 12 names in it.

"I started thinking about the old days, and all the memories attached to those names, and wondering if my friends were feeling the same, if they were sitting somewhere with a manicured hedge drinking crisp white wine, or whether they'd hung on to the past somehow. And I just thought: 'why not?' "

A lot has come from Wallace saying: "Why not?" Back in 2001 he posted an advert in a newspaper with the words "Join me" and was inundated by the response, although no-one knew what they were joining.

That led to the book Join Me: The True Story of a Man Who Started a Cult by Accident. Then, after meeting a stranger on a bus, he decided to "say 'yes' more", leading to a book, Yes Man, which has just been made into a movie starring Jim Carrey.

But Friends Like These is his most personal book to date. Beginning with Google and Facebook, it turned into a physical journey to the places where Wallace grew up: Dundee, Loughborough, Bath, Berlin, and a nostalgic journey into a world of BMX bikes and The A-Team. "It is a lot more personal. It's not just a bloke doing silly things, it was a little journey of discovery.

"I think nowadays we get away with more for longer. When my dad was coming up 30, he was about to have me, he had a proper job, all that kind of stuff. We're allowed to dance around and be silly for a lot longer. Eventually you realise somehow you can't do that, it's time to grow up, take things more seriously, do some wallpapering. It helps to know that growing up is universal."

He admits to an anxiety at the start that all his old mates would work in IT. Where's the book in that? But they came up trumps: one is a German rap artist, another claims to have solved the secret of time travel, a third turns out to be a member of the Fijian royal family.

"I knew there was something about his family which was a bit special. I remember going home with him one day in Loughborough after being out on our bikes and there was a limousine outside my house with a Fijian flag.

"The Fijian ambassador and the Minister of Defence were inside eating finger sandwiches with my mum. Now I find out that not only is he minor royalty, he's got his own village!" Wallace grins. "I was always attracted to the slightly unconventional kids."

Didn't he worry about what his old pals would say when their mate – now coincidentally a television personality – turned up on their doorstep after two decades? "There were a couple of moments, early on, where you think: is this a bit odd, turning up at someone's house after 20-odd years and saying 'Hi, are you coming out to play?' But the minute they put you in context, there's an aura of fun around it. And you've got so much to catch up on.

"I'm still massively in touch with them. Anil texted me this morning, I phoned Cameron over the weekend, Simon's coming down next month, it's just great! You share so much with them, and the stories don't stop coming. It feels not just like you're making new friends but you're rekindling old friendships, so you score two on the friendship chart." Lots of us think about tracking down our old mates, but not many actually do it. "People sit idly at work and look up their old friends on Facebook or Friends Reunited, but they don't do the next thing which is to say 'hi', and meet up, they just sort of spy," says Wallace. "Why not make the leap and have a pint with an old friend? I call it face-to-face book."

This is Wallace through and through, relentlessly optimistic, endlessly likeable. I ask if his talent for making friends owes something to the fact that his family moved around a lot. "I've always had to fit in and make friends fast. I guess being able to bond with people was a survival thing. But the friendships I made and make are always pretty strong. I love my friends, and feel very close to them."

When the Berlin Wall came down, his father, a specialist in modern German literature, accepted a post in Berlin. Even before that, the family phone had been tapped for years because of his suspected connections to dissident writers.

"My mum is just the sweetest woman in the world, and the day we left Dundee, she actually picked up the phone and said 'Hello, just to let you know we're moving now, but the people who've bought the house are radical CND activists, so you can probably keep the bug going if you want'."

After leaving school, Wallace did a media course and reviewed video games before securing a place on the BBC's coveted trainee producers scheme. He worked on shows such as Dead Ringers and The Mighty Boosh, before packing in the day job because he "didn't want to sit in an office all day".

It was the era of the comedy quest, the bloke-doing-rather-silly-yet-occasionally-meaningful-things genre of light entertainment. After working with his former flatmate Dave Gorman on his madcap quest to find as many people as possible around the world who shared his name, he soon had an eye out for a project of his own.

"Tony Hawkes (writer of the best-selling Round Ireland With A Fridge] was a pioneer, but I think it's just the way boys are. Around the World in 80 Days, although it's fictional, is that kind of boy's own adventure – somebody taking something to ridiculous extremes."

So there was Join Me and Yes Man (both successful books), and How to Start Your Own Country (a BBC2 series). Taking it all at face value, he was an ordinary guy, bored with sitting around in his underpants playing computer games, who had an idea for a prank which took off. Being more cynical, he has used his media skills and contacts to capitalise on a carefully conceived plan. The truth is probably somewhere in-between.

Either way, it would be harder to invent a more suitable media personality for the 21st century than Wallace, who can equally well front a quiz show (School's Out) or write a documentary (the next is "Where's My Robot?" for BBC's Horizon). Who can turn out books and write newspaper columns and do interviews, all while coming across as unendingly good-natured. Either the man has an uncrackable professional facade or he really is that nice.

Hollywood loves him too. A "star" is poised to sign up to make the movie of Join Me, and Friends Like These was optioned by Miramax even before it was published. "So you know that pub discussion about who would play you in a film of your life? Now all my friends can really do that!"

Yes Man, directed by Peyton Reed (Down with Love) and starring Carrey (as Wallace) and Zooey Deschanel (late of The Happening) as Lizzie (his then girlfriend, now wife) moves the action to LA. Wallace has seen it, and loves it. "It's funny, it's warm, it's romantic, it's really well directed. I think it will go down well. Although he falls over more than I did." Still only 31, he's careful to play down his success. "I've never really had a ruthless ambition. I've had a polite ambition, because I want to do things I enjoy and hopefully other people will enjoy.

"But you don't write a book to get it made into a film. It's not like I'm JK Rowling, millions of people don't buy my books, it's not that big."

I venture that having Jim Carrey playing you in the movie of your life is really quite big. "That's cool. I always forget about that, it's part of your life and you forget that it's as crazy as it is."

Maybe this is because he manages the craziness so well. He has delegated the running of Join Me – now a worldwide "karma army" 10,000 strong, intent on doing good deeds every Friday – to his mum.

"I recently had an e-mail from a lady in Nova Scotia. She went to buy a coffee one Friday from her usual drive-thru coffee place and since she'd just read Join Me, she also bought a coffee for the car behind. And they did the same, and the next car did the same. It went on for 21 cars.

"I think we've just found the opening for the Join Me movie."

• Friends Like These by Danny Wallace is published by Ebury Press, priced £11.99.