To me, it depends on how specific the idea is.
If I posted about this spell I came up with for a story, complete with a name and exact description what the spell is doing, and someone used this exact spell in a story without asking me - I'd be a bit miffed.
But if I posted about how one day I would want to write a Bill/Blaise fic with dragons, and then someone wrote such a story - that to me is perfectly okay.

I guess the best rule of thumb is: if in doubt ask.