I'm biased so I voted for "other".
Social Networks bring people together but they don't reinforce any kind of relationship.
That is to say, a person can have 100 "friends" but it's doesn't mean they are "friends" with 100 people in the traditional sense of the word. A site like Facebook is just a facilitator. It's a means to a connection to an online entity and as such, the term "Social Network" is a bit of a misnomer. To me, anyways.
When I scroll down my FB or Twitter feeds all I see are mostly cut and pasted or perfectly crafted snippets of some funny/awkward/interesting though or activity; mumbled and disseminated - not unlike standing on the roof of your house and shouting "I loved this Jello-Pop I had" to anyone within earshot.
The worst offenders are the social marketing hounds. But that only affects you if you follow/connect with them.
I guess you can't blame social-marketers for cutting out a little piece of the pie for themselves and diluting that 'sorta' interpersonal part of social networking. It's less about who you meet and more about how many people you can reach. That frustrates me a little. But, that's evolution. Can't stop it.
It's funny because I too believe the best social networks are forums. At least on a forum it stands to reason that there are going to be people around that you know and people that you don't know. If you're new then people have to get to know you. Isn't that the whole point of a social network? It's not about being in a controlled environment and giving only 'certain' approved people passes. That's a "Totalitarian Network". The forum is the true Social Network. It's open to everyone and anyone can talk to or learn from anyone else and as such, is free'er and more socially conscious which generally facilitates more meaningful and longer lasting relationships.