Social networking has what I call an indirect effect on search engine rankings. The purpose of social networking is to build exposure for your website/blog. As you build your network you are building a following. As your network grows so will your website traffic. This will have a positive impact on your search engine rankings...but it takes time. If you want to really improve rankings you need to have several irons in the fire, social networking being one of them. You may also want to consider PPC advertising, link exchange, etc.
there is a lot said on this topic. search this forum - you can find some nice guide and maybe ebooks on it
i suggest you use a different method for increasing traffic on your website rather than working with social media networks
True - this is part of Google's algo - if it sees a lot of traffic going to your site - it will move you up in the SERPs. You can use Twitter and FB and Youtube to accomplish this (this is just one way).
A great way is to make a fan page, link it to your website and get lots of fans to your page. Your fan page will show up in the SERPS, and your webpage will increase ranking as well.
Social Media Optimization can help you in generation of website traffic but Search engine ranking depend on do following links.
Most of the bigger social sites provide links that are not dofollow so theri links in theory should not help your SERPs. Watch out for any dofollow social lists that you find. Most of them are outdated.