[Disasterpiece]

social networks won't replace a forum in the near future.
They do replace old-fashioned messengers like msn, icq and the like as well as irc maybe, but as a whole they were filling a void rather than advancing what was already there.
Forums are good for collecting information and bringing it in an order.
Social networking are more focused to short conversations, sharing of opinions, media, etc. It's unordered and will be forgotten after 48 hours.

Facebook login is a good way to catch additional users, who are lazy or to collect more users who wouldn't have used the forum with a conventional login. Maybe useless imho, doesn't really offer enough benefits to be really useful.

The like functionality is good for making your forum known throughout the social networks. Same for the publishing method. It just looks better than someone posting a link somewhere.

I myself don't use any of that stuff atm, but that's because I mainly administrate support forums where it's focused about topics and help within an productive community, not that chit-chat kind of thing.
Use it if you feel your community may benefit from it, not if you only want to have it and brag about it.