Need a little bit of advice to get things going?

I met this girl that I find quite attractive at a party a few weeks ago. Since then I have added her on my social network site. I have had the intention of chatting with her on there, however, I never see her on when I am on it although I can she she is actively using it. So I am not sure how to start up a conversation with her considering that I do not have her number and she is never on her social network site the same time as me. Well, there is the option of sending a message to her inbox, however, I feel as though talking with her using pop chat would be better, especially for the first conversation.