I never correct grammar because mine is terrible. I'm 'clever' and educated. I write for a living. I know the difference between where, were and we're but my phone doesn't, nor do my fingers when I'm typing a post on an informal internet chat forum, at speed, for entertainment.
I do think the people who correct grammar on serious posts are twats but the ones who do it on posts like 'AIBU to think wimmins shuld stay in the kichen?' I find funny.
And Facebook language makes me cringe. Wtf does gawjuss mean? And why use it? It has almost as many letter gorgeous, anyway.
I wouldn't tell someone who comes on to ask if their bubs luks gawjuss they're a twat but I would quietly think it.