I feel good with twitter for simplicity and Facebook for features. More over both have high success rate for users.
There is nothing called 'best social networking site'. It depends upon the choice of individuals. SO me may favor Fb,some may go for Twitter or some may say that they like MySpace. Choice of social networking site , for business purpose, must be done according to your objectives and target audience.
You can increase your benefits from twitter in this way: When you follow people, people will also follow you back. However, I will advise that you follow only targeted people. For example, you can search for people asking for a particular problem on twitter by typing "question/problem? -http -www" (for example" can't sleep? -http -www") at search.twitter dot com. It will show those looking for solution on that problem on twitter, then copy their user-name, log into your twitter account, copy their username one after the other into the find box, click find, then follow them. They also will follow you back. Then sometimes (not always) tweet about a product that solve their problem or reply to their tweet where they are asking for solution by tweeting about the product (with your affiliate link) that solve their problem. This way, you will be following targeted people, creating relationship and at the same time making money.