Tough question! There are a lot of famous people I wouldn't mind hanging out with or getting to know, so the list could potentially be quite long.
I guess the most interesting ones are as follows:
Steve Buscemi & Paz de la Huerta
I'm a HUGE fan of the TV show Boardwalk Empire and as a bonus maybe he'll introduce me to the rest of the cast too. I'd love to meet Paz de la Huerta (who played Lucy Danziger). She was also really good in the movie Enter the Void, plus she looks like a bit of a party animal!
Johnny Depp and Hunter S ThompsonSadly not possible since Thompson died in 2005, but the two were good friends and Johnny Depp played Thompson in a movie called Fear and Loathing in Las Vegas which was based on one of Thompson's most successful books.
Marilyn Manson & Trent ReznorKeeping up the trend of duos in this list, I'd be interested to spend some time getting to know these guys because they're the musicians that most of my pocket money went to as a teen, so it'd be good to get my money's worth :)
I'd also be interested to see whether they've kept up with their 'wild' personas that made them famous, as has growing older mean they've calmed down a bit?
Historical figures
If time travel was possible, there'd be loads of people I'd like to go visit and make friends with. Everyone from Jesus to Julius Caesar would make great Facebook friends at least!
Fictional Characters
How cool would it be to meet characters that only exist in books or films? I'm sure I'll think of someone more poignant later on, but for now the one that sticks in my head is Peter Griffin from Family Guy!
My Ancestors
I don't know much about my family history and lineage etc.. So it'd be fascinating to travel back in time several generations and see where these amazing genes originated from!
Amanda Bynes & Miley Cyrus
Ok, I know... a bit of an unexpected choice!
But I have a bit of a soft spot for celebrities who go 'off the rails'. I'd like to think maybe I could be a calming influence on them. Also, it'd be fun to put all that notoriety to good use by working together with a celebrity to do something constructive like recording an amazing record or developing an iPhone app that goes viral!