When I read something that's misspelled, I automatically think the person who wrote it is stupid.
I can't help it.
Of course, people make typos on Facebook. So I'll forgive if they automatically correct themselves. But when they spell something completely wrong and don't fix it, I think they must not know how to spell it. If you do this once, I don't write you off as stupid forever, but if it happens pretty often, I definitely think quite a bit less of you.
This is absolutely not limited to Facebook.
I read an article yesterday that was assigned for a class, and it had misspellings and grammatical mistakes. It was a blog entry, but still. It was assigned reading. I expect my reading assignments to be well-written.
Fliers, signs, etc. are whatever to me, I've gotten used to those types of things having hilarious mistakes.
But when you're posting something for everyone to read, don't you care about getting your message across correctly?