Hmm, dunno, it's a hard one.
I have recently got in contact with a lot of old school frineds on FB too, one of them said to me 'do you remember so-and-so?', and I did, he was a lad that was quite geeky at school and didn't expect anyone to remember him, she then told me she was in a relationship with him now, and apparently when she told him we were back in contact he said he used to fancy me like crazy at school.
So in one way, I think it's just silly joking around, like 'Ohhh, I remember blah blah, in fact I had a huge crush' (kind of like I might say about Edward Furlong in the terminator 3 video!) but if it has made you feel uncomfortable then it's not fair.
I'd not say anything for now, but keep an eye on his fb traffic , it may be just be an innocent one off, or use your hormones as an excuse to get all tearful about how shit you feel and then tell him in that conversation, something along the lines of 'I know it's stupid of me, but I saw a message to so-and-so about how you used to fancy her and she looks all slim and pretty and it made me feel really insecure'.
Depends if you feel you need to mention it to feel ok about it.