What makes for a Facebook friend, anyway?

How many friends do you have on Facebook (assuming you're on Facebook)? A hundred? Five hundred? A thousand? How many is too many?
Late-night host Jimmy Kimmel, for one, suggests that if you've cracked a thousand, maybe it's time to start "cutting some of the friend fat out of your life."
The bloggers at AllFacebook caught Kimmel's monologue Wednesday night. "I see people with thousands of what they call 'friends,' which is impossible," Kimmel insisted. "You can't have a thousand friends."
How do you tell a real friend from a Facebook friend? Easy, Kimmel says. "Let's say on Friday, post a status update that says, 'I'm moving this weekend and I need help.' The people who respond, those are your friends. Everyone else isn't."
Or, here's another example: "A friend is someone you have a special relationship with, it's not someone who asks which Harry Potter character you are."
During his rant, Kimmel went so far as to announce a new holiday: "National Un-Friend Day," Nov. 17.
"Friendship is a sacred thing," said Kimmel (who looked like he was only half-kidding), "and I believe Facebook is ruining it."
There are, in fact, actual limits to how large your circle of Facebook friends can grow. Facebook will cut you off after your 5,000th friend—purely for "back-end technology" reasons, as the New York Times notes, not because site administrators think you're defining "friend" too liberally.
But is it really impossible to have 1,000 friends, as Kimmel semi-seriously argues?
Yes, it is, according to evolutionary anthropologist Robin Dunbar, who writes for the London Times that "our brains aren't big enough to allow us to have deeply meaningful relationships with more than a handful of people."
In fact, 150 is "the average number of people to whom most of you send Christmas cards," Dunbar says.
A hundred and fifty friends, eh? Well, I'm about a dozen Facebook friends over Dunbar's limit. Looks like some of you have gotta go.
Of course, this whole little debate forces us to ask: What is a Facebook friend? Should you "friend" only those who are true friends -- those people whom you "can have a relationship with involving trust and obligation," where "there's some personal history, not just names and numbers," as Dunbar defined friends to the Observer earlier this year?
Or is Facebook exactly that: a contact book of names, faces and numbers for close friends and mere acquaintances alike?