What would you think/do if another guy left this message on your significant others Facebook page?

[J]

I would think: He likes her.

I would do: Nothing. If you react then you're going to look like a jealous control freak.

If he continues to put little flirty remarks on her page or insinuations of his feelings for her, it is HER job to do something about that, and to set the boundaries with this guy. If she's mature enough, she'll know that the right thing to do is to not feed into this guy's flirtation and will do so on her own accord. If she's not, she'll probably eat up the attention and bask in the glory of knowing more than 1 guy likes her, and she'll probably try to delicately walk a thin line to try to not piss you off, but also not completely blow him off either.

Watch and see which girl she is.