I am not fan of these internet social networks either. In those sites, people expose theirselves in a way to attract others attention for what? I believe that facebook/orkut and stuff like that are a very easy way for cheating.
I was involved with someone that used these sites a lot and I was cheated many times.....it was sad because I knew he frequently used these sites and he never wanted to share any information with me....I believe that in his "virtual life" I never existed and he was free to "date" whoever his feels like...(which in fact he did)
Maybe your boyfriend is trying to be quite honest with you by letting you watch him do it. Besides that, people know that he is with you and that he is proud of that! It is a good sign! I know it is hard now but try to relax and dont think much about it. Good luck!