This feeling of insecurity is a very common problem with these "social sites" when old flames pop up, long lost friends of the opposite sex etc etc. A work friend of my partners opened a facebook account recently and her husband was not happy about it at all. So often these days we hear of relationships being ruined in similar circumstances. I would say that there is a touch of paranoia in your concerns, reading the conversations, worrying about an "x" to be honest I think that is a natural reaction, I think those kind of concerns would cross most peoples minds. If it worries you talk to her about it, dont go making demands of her such as telling her to get rid of him on facebook though, that could just make matters worse.