I would guess that the Facebook scrabble is Flash based. I don't really get the point of social network sites, so I have no direct knowledge of what the various Facebook games are, I'm just basing it on some assumptions made from what I know about how several other sites do similar things.
If that's the case, then it's likely some bug in either Safari or Flash, maybe some kind of unintended interaction between the two. Given that Jobs has been pretty vocal about his dislike of Flash, I can't see that this is going to be a high priority to fix. But since Flash craplets are basically a whole different kind of beast from your bog standard web page, things may not always work as expected when they are involved. Not much you can do but try some of the other browsers out there like Firefox and Chrome.
My computer is a mac OS X version 10.6.3. I've noticed that my command+ shortcut to 'zoom in' doesn't always work. If I'm playing scrabble on facebook, it NEVER works. I have to go into VIEW, ZOOM IN, and then it works. And even after I do that, it randomly zooms out. Could this be a glitch in facebook scrabble? I think this problem has happened in other applications as well. Shouldn't the command+ shortcut always work? Please help.