In a good relationship, partners talk to each other about what they would like.
There is nothing wrong with you asking to be included on his Facebook, particularly if you can explain why it is important to you (and it shouldn't be because you don't trust him). There is a big difference between "asking" and "demanding"; for one thing, if you ask he has the right to say "no". It is healthy for people in a relationship to have some space from each other.
My advice would be to talk openly and respectfully. And, make sure it is a two way conversation. You should say what you want in the relationship clearly (understanding that no one gets everything they want). But you should also listen to what he wants and needs.
In good relationships, both partners work to understand, support and trust each other.
If your goal is to be included more in his life (which is a fine thing to want), then you can ask for this. Again, remember that this is a conversation that needs to be worked on... it can't be a demand. I would certainly respect it if my girlfriend told me this. If your goal is to pry into his life to keep him from having secrets, it is a much different thing.