This falls into the category of "amusing personal anecdotes" rather than "rigorous industry analysis". But it also reflects on the complexities of getting user experience right, and mapping telecom and mobile services onto the way people actually live their lives.
One of my close friends is, for want of a better term, "a bit of a player". Trying to keep track of the various females in his life is a fulltime job. One thing is clear though - FaceBook is pretty important to him, as is SMS, MSN, Gmail and obviously mobile voice. He doesn't have a landline, and his Blackberry is strictly work-only.
But although he uses FaceBook as a hub for his busy social life, he does not really appear bothered (yet) about getting mobile access to it. He'll ask people he meets "Are you on Facebook?", remember their name or email address, and add it later via the PC.
I'm the same - even though I do look at FaceBook on mobile, I think I've only ever once "added a friend" on a phone - and that was in the relaxed confines of a conference, rather than in a noisy bar. (Note to readers: I don't have work contacts on FB, so please don't request it).
Conversely, if my friend wants to swap phone numbers, he'll type in the number and "give the other person a missed call" so they get the caller ID. Sometimes he'll just hand the phone to a new acquaintance and invite her to type her own name & number directly into it. He might also add an email address or IM nickname.
I've seen various people suggesting that mobile "will become the new social network". I'm not 100% convinced, especially because of the incumbency of Facebook / Myspace / Bebo etc. On the other hand, whoever gets the "add friend" experience working first is probably going to get a huge advantage.
When I see my friend say "Hey, you're really cool, we should meet up some time. Put your Facebook ID in my phone" I'll know it's really going mainstream...