This is very true. However for marketing purposes, Facebook is not enough; there are other important social media which every online business should use for driving traffic to their website. First of all, Twitter is maybe the most important micro-blogging social media available and therefore an invaluable tool for website promotion. Secondly, LinkedIn is an extremely important social media for connecting with other businesses and professionals.
From what i've seen it will take lots of years before facebook can be replaced by other social networking sites.
I believe other social networking sites have no match to Facebook.. all are focusing in in their facebook nowadays.