Database design for a social networking site
Try having a look at Database Answers in particular the data models. They have several different designs for various systems. This one is for a social networking site which may give you an idea of what's required.
You may want to search on SO for other social network database questions. I found this one that had a link to flickr showing a schema which appears to be from Facebook.
Your database design will be based around your system requirements. Without knowing exactly what you are trying to achieve, it is difficult to give you the best design.
You can create a separate table for follower/ followed relationships. So, when x follow y, create an entry with follower_id = x.id followed_id = y.id.
You can query the relationship table to look for all the users x has relations with by select * from relationships where follower_id = x.id or vice versa.
When/if x un-follow y, you just have to delete the entry you originally created.