There are so many groups on Facebook so it's easier to Market to your niche. I'd go along with facebook.
I think face book and twitter is best of me... so that.. thanks for sharing...meet again... Facebook is the standard, if you want to make a presence in the social media sphere, you have to have one. It's the de facto standard, and it's useful for keeping up to date with the social lives of your friends as well as keeping tabs on social movements and news. It's also a great way to present a more laid-back, casual side of your personality, especially towards your business contacts and colleagues.
Social Networking sites are websites that allow users to connect with each other and post information to each other. Some big social networking sites are Myspace and Facebook for example.
the total player of a game in facebook (farmville) is more then the total member of twitter...so u can imagine how big is facebook. for marketing purpose facebook is best