If somebody "friends" you, it gives the both of you access to see things that the other has posted to their profile. For instance, I save all my pictures that I post to the forum in the Album feature of my profile. Only people who've "friended" me can go into my profile and peruse ALL of the pictures that I've uploaded.
There is also a Message feature that you can communicate privately with people that you're "friends" with, separate from the Private Message feature of the forum.
The Wire feature looks to be a twitter-like function that you could write things and everyone you're "friends" with would be able to see it. That's pure speculation, I haven't played around with it to figure it out.
The bottom line, to me, is that it looks like the software is attempting to emulate some features of Facebook, but it's never taken off because the vast majority of people here are content to interact on the forums.