Is it okay to add a girl on Facebook who rejected me?

I was at this party the other night and saw this girl who I really liked. I struck up the courage to go and talk to her and after talking for a while we had a lot in common. She said I could add her on Facebook.

I then asked her a bit later on if I could take her out for a drink some time at which point she replied that she had a boyfriend.

Would it be okay to add her as friend on Facebook even though she rejected me?