As I said in my post, I'd include forums in "social." Facebook has the graph, but you could construct a weaker form of that graph using any of the popular community-focused sites.
Not really. Facebook and other social sites nowadays all rely on one true personal identity. On forums on the other hand you can be whatever, multiple times, and have a discussion with yourself (even a meaningful one).
I have at least one friend with multiple Facebook accounts, which he has talk to each other as a joke. Much like Facebook, most of the significant interactions on forums happen between permanent, established accounts (4chan could be the exception that proves the rule, as everything on it is arbitrary and real projects are organized elsewhere).
Facebook wants to portray itself as a one-person-one-account service, but the lack of extra accounts is largely because they've obsoleted most of the reasons people would create multiple accounts.
