That depends on the audience you're targeting really. Do you expect people to find you on Facebook? Then it would make sense to offer Facebook login. Nearly everyone has a Google account so why not offer that option?
It only takes about 10 minutes to set them all up so it's not going to be a big bother to add them. As far as whether or not this encourages people to sign up I have no data on that. However, I have to imagine the option to sign up without having to create yet another account would be enticing.