To be clear, I notice that mistake frequently...and there are plenty more that are just as common. Sometimes I laugh; sometimes I cringe. The reality, though, is that (for the most part) the people that don't know, don't care. Pretty commonly, pointing out such things will get a defensive response..and earn one the title of Keyboard Commando, Grammar (or spelling) Nazi, etc.
Personally, I would love it if people used correct spelling, reasonably correct grammar, and decent punctuation. Heck, I'd like it if some of them could form complete sentences...or break that 200 word mess into multiple sentences. Those things would be nice; but I choose my battles - it helps me to not get bogged down in No Win stuff, and enjoy my time spent here.