All the three sites Facebook, g+ & Twitter are best Social Site. Every site has its own importance, own awareness.Facebook and Google plus are both pretty awesome, but Facebook is leading the market.
As per My view, Facebook is good.Facebook that facebook has a bigger audience.Generally People are more active on Facebook, so if contents are good it will reach to maximum people hence Facebook is good.
And one more thing, Facebook contain more features then twitter .Facebook has a option to make groups with your friends and twitter does not contain this feature. Finding friends is easier on Facebook than twitter.So prior to above benefits, I would said, Facebook is the best social networking site ever.Facebook is the most widely used network, but Google+ isn't far behind either
Marketing on Facebook is far more expensive than marketing on Google+.
At the conclusion, The best network is the one you can utilize best with regards to your cost constraints and your product.