1) Globalization - the internet has given us the opportunity to meet and interact with people from all over the world. So, our choice of partners is no longer limited to the local singles with whom we can interact face-to-face. The internet is a cheap, efficient way to branch out and find someone who is a perfect fit for you.
2) Individualization - people are defining themselves more based on their own interests and not the interests of their community. This effects partnerships because we are now looking for someone that specifically fulfills our needs and matches our personality, rather than someone who can help us contribute to society more. It is much more difficult to find someone that is an exact match to an individual than it is to find someone that could simply help in the fields, for example. So again, the internet gives us a much broader pool of people to choose from.
I agree that anonymity is a great reason to use the internet for dating, too. But that doesn't have much to do with social changes.