Found out she is seeing others and I don;t know what to say

I started seeing this girl nearly two months ago, since the start we've said we would keep our options open and see how it goes. The other day I realised she was seeing two other people in the same situation. Even though I hate this i still have very very strong feeling towards her and don't know what to say to her about it. I've also noticed that I'm the only one that she posts stuff on facebook with... pictures of the two of us etc. I really want to tell her that I want to move into a serious relationship with her, I just need some advice on how to do it. Thanks

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