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.