The first reason is obviously back links. If you are constantly writing and commenting on forums, you can provide back links to your own site. This will bring additional traffic to your site but more important it will attract search engine bots. Back links increase your search engine ranking.

The second most important reason is that social media sites like    Facebook, MySpace and Twitter provide fresh content every day and therefore attract search engine bots. By linking to these sites you will increase your exposure to search engines.

Your sites search engine ranking depends on many factors. Social media are an important part of your SEO strategy. Moreover, posting on forums is free. You must find the right forums to post your comments. By creating sufficient back links you improve your overall search engine ranking.  Sites like Youtube are a favorite hunting ground for search engines. By posting a small video on this site with relevant keywords can do wonders to your site visibility. Ultimately, you are looking for traffic and social media sites can do this job effectively. By focusing exclusively on keywords and site optimization you are   likely to miss out on some great opportunities.

Twitter is another site which can ramp up your traffic spectacularly. By cleverly associating your tweets with some happening news can be rewarding. You must create a Twitter strategy specific to your needs. Tweeting can be monetized well, if used  in combination with your other social site activity and website.