I would walk away and keep walking. I don't believe her. All that crap she wrote on FB are just words. Actions speak louder. Its not like she had been drugged and didn't know what she was doing. She was completely sober, she met up with this guy alone (who she likely had feelings for in the past and probably still does) and she kissed him. Shes not an innocent victim so don't allow her to make herself out to be one. Its good that she was honest and told you that they kissed-at least you know you can trust her to be honest with you but can you trust her around any male friends again? No! Will you be paranoid every time she goes anywhere without you in the future? Yes..
Just block her on FB and move on. Cheating is cheating. Some people say "its just a kiss, could be worse" but I think it all hurts the same. Whether its emotional cheating, cyber cheating, just a kiss, a bj, full sex, prostitutes-whatever. It all hurts equally and once the trust is broken-its impossible IMO to get it back again.
"Don't ask a question if you can't handle the answer".