I have used a lot of social media as a marketing tool for the various companies for which I work. I've used forums and other tools, to get the word out about our companies and what they do.
In the time I've been doing this I've found there are two kinds of people on the forums and other sites. There are people who are truly there to contribute and market their businesses, and there are people who are there solely to market. The ones who contribute and market a bit on the side usually have immense credibility. The ones who only contribute when they want to push their own product or service usually lose credibility.
Are you more likely to do business with someone who contributes to a social media site before they market their own product or service, or does that not matter to you? Do you just assume everyone is there to market?
Bookmarks