[mh-]

I'm sure it's already rendered to a static presentation-level (HTML/template language du jour) form at that point. That wouldn't work.

[mibbitier]

Just put some more javascript in there to deal with it. I'm sure it's not the hardest problem in the world.

[engtech]

Great, you've hidden the message with javascript, but now it's still in view-source and in search results for bots.

It's also still visible if someone is using noscript (actually, I have no idea if Facebook works with noscript, probably not).

The solution you are suggestion doesn't solve the problem and injects more corner cases.