Such people r called Grammar NAZIs. I met few of them in Deviant Art and all of them are super rude. They talk like they're genius. They make sure u feel guilty about ur mis-usage of English. As a self-published gay writer (esp coz English is not my primary language), honestly, grammar Nazis discourage me sometimes...
It's VERY RUDE! It's similar to telling a stutterer that he/she stutters. I mean, he/she knows he/she stutters. No need to point out the obvious.
I myself usually type a lot of typos coz when I type fast - my vowels always come before consonants. For example I type 'eher' for here. Other time is tense problem coz the confusing past vs present, or simply coz I'm not paying attention.
When talking to friends, I NEVER correct them. I understand their typos. If I don't, I ask politely.