There is a lot of talk at the moment about Social networking sites (Facebook Twitter). Do really they help at all with google ranking or are they just a marketing tool?

Not a whole lot really. Twitter I wouldn't imagine would pass on much relevant or authority. I trust Google's search quality team are dropping the authority passed from Facebook etc as it's largely open to spam.

The thing about social media is it's about engaging and networking. While it mightn't directly increase authority/PageRank (who cares about PR?), it's an important source of traffic and business for many people.

If you are a consultant or professional, you may be more effective engaging directly with people instead of visitor numbers.

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I can say that Social Networking like Twitter, Facebook, Myspace are good for marketing and for promotion. You can get traffic but not exactly helping you to increase SERP.

Facebook, twitter are very much famous social networking site. hi5 is old and not used nowadays.