ehhh, I don't see people mistake 'than/then' and 'were/where' as often as you seem to- and certainly wouldn't let it mega frustrate me.
The internet is in fact full of people of all ages, regions, etc. Some may be on the phone and using an auto feature that fills a word based on a few pushes, so it's not always necessarily someone literally making a mistake but rather going for ease of quick use.
In the end- who cares. If it was your GF/BF doing it constantly then I get bitching, but a fraction of the people online?