The only time I try to make a habit of correcting spelling and grammar is: when I'm writing essays or fiction. When someone has asked me explicitly to correct their spelling and grammar. When I'm getting paid.
But on the internet at large? I generally try to avoid it, because it would make me look like an insufferable jerk.
But why would that be? Well...
- A lot of people on the internet don't speak English as their first language (or most? There are fewer English speaking countries than non-English speaking countries). You can't tell from a picture of someone where they come from, and it's entirely likely that they learned English as a second language. How many languages do you know? Are you perfect in your second/third/fourth language's spelling and grammar?
- Lots of people have learning disabilities which translate to difficulty with things like spelling or more obscure conjugating conventions. Again, you can't tell by looking at someone if they have a learning disability that affects their writing skills. And it doesn't mean they are stupid or slow.
- Not everyone - even in North America - has the privilege of an education that taught them the difference between to and too, or there/their/they're. Access to good schools is not universal. Many public schools don't have the resources to meet their students' needs, and many children and young adults are in situations that make it difficult for them to attend school - usually poverty related. And for the record, not having access to an education does not make a person stupid.
- Do you know how often my phone autocorrects to the wrong their/there/they're? Or "of" to "if" and vice versa? If I'm writing a long post while on the go, I don't always catch it. Typos happen. It's not the end of the world.
- Places like Facebook or reddit or whatever are conversational places. I don't know about you, but when I talk to my roommate or my boyfriend I don't sound like I'm dictating a thesis paper. There are conversational conventions in English that don't follow strict grammar rules but are understood to be just how people talk. It's informal. It's comfortable. As our interpersonal relationships move more and more to online spaces, you should not be surprised or annoyed to see these same conventions transfer over to non-academic, non-professional online spaces.
- Dialects exist. English has many regional and social differences in spelling, pronunciation, and grammar. British English is different from Canadian English is different from American English is different from African American Vernacular English is different from Scottish English is different from Newfoundland English is different from academic English is different from... you get the idea. None of these dialects are better or worse, more or less proper, more or less intelligent sounding than any of the others. They are just different.
- If you're involved in an argument/discussion about whatever it is you care enough to get into an internet argument about, and your only rebuttal is "*they're", you're literally (not figuratively) telling everyone who can see your comment that you couldn't come up with a valid argument to support your side and have basically resorted to implying the other person is an idiot for using the wrong version of their/they're/there (or whatever) - which is likely attributable to any of the reasons I've already listed above. Oh, irony.
If you use the phrase "grammar Nazi," you're being an anti-Semitic dipshit and minimising the deaths/ethnic cleansing of millions of people by comparing a penchant for correcting grammar to being a Nazi. Don't do it.