We're friends but he doesn't add me on Facebook?

[Hey]

A guy in my Biology 124 class who is very outgoing and friendly adds his other friends that are girls except for me? He's friends with a lot of people including myself but unlike them, he hasn't added me on Facebook. He sent the friend requests I'm sure because the same girls whom he's friends with are also my friends and they're known to never send requests unless it's to their family members. I talk to him in school and at work since he would always open up conversations. Does this mean anything? Thoughts on this guys? He has about 328 Facebook friends
He knows me, full name because he is the teacher's assistant for the class and sometimes he's the one correcting the tests with the answer key and handing the papers back. The girls whom he's friends with don't like to add people unless it's their family members. They have about 30 friends which are 95% family