I hate social media...
facebook - super annoying and violates everyone's privacy, but sure. They have a lot of users and once a page is setup takes minimal effort to maintain.
twitter - annoying little tweets (who ever thought it would be ok for a real mans man to utter tweet and not be made fun of?) about things nobody cares about. Also has a lot of users and can be setup to autopost tweets about each new story, etc.
google+ - does a single person actually use and like this?
overall these are easy ways to keep members updated about the site, but how many new members will they really attract? In my opinion not that many, but there is simply no logical reason to not have Facebook and twitter accounts.
other things that were mentioned: tumblr, youtube and reddit. Tumblr I have come to enjoy personally. It's great for pictures and reblogging is easy and can spread those post easily. When it comes to content though, not so much in my opinion. Youtube, as annoying as a google controlled youtube is, it is actually like a search engine in its own way and could be an easy way for people to stumble on content from people here. There is the key though....people. Someone would need to be posting videos in behalf of the site. Vlogs do present an entertaining medium but they take work, they remove anonymity and it's not exactly spreading the content of this site, which is mostly text based. Reddit, as much as I hate to admit it has become an extremely large online community. It could very likely be one of the better ways to attract new members and attention to stories here. It would require dedication on the account of members from here to promote the site there though. It wouldn't be easy but could pay off.
overall one of the better ways and more traditional is simply content based. The more posted here, the more browsers will return this site and the higher ga will be in those lists. Also the more authors post stories to other sites (including nifty, despite their awful policies) and either include links back to here or keep portions exclusive to ga, the more people will be drawn here to read. In the end ga will be as popular as the people who choose to call it home. Thus talent and quality people should always be the driving force here...
of course there is the whole domain name discussion, but I don't think that fits with social media.