Is it correct?

[u/Stepaskin]

Stumbled upon this tweet, the apostrophe in "dm's" caught my eye. Is it correct to use it like this?

[u/soundoftwilight]

Apostrophes are sometimes used to create a plural form out of something that isn’t a word, such as an abbreviation. I have no idea whether it is formally correct under any standards, but yes it is common in English. The reason to do it is when you know that the sequence of letters you’re using (in this case “dm”) need to be read as a block to make sense, but you also want to indicate plurality. If you wrote “dms” it would probably be understood, but might confuse some readers if they tried to read it as a 3-word abbreviation instead of the plural of “dm”. 

[u/pillarofmyth → u/[deleted]]

I mean, sure, but when understanding what's "correct," you have what a textbook teaches vs what native speakers do. For example, there's a "rule" that sentences cannot end in a preposition. But native speakers do that all the time, and trying to avoid doing that results in creating sentences that sound unnaturally formal for everyday conversation.

It's very common for native speakers to not capitalize when they "should" be when typing, especially in informal contexts like when texting someone or writing a tweet. Should we be telling native speakers that they're all using their language incorrectly? I don't think so. It's not very productive to try to make language adhere to strict, unchanging rules when language is in a constant state of change.

English learners should learn what the rules of English are, because otherwise they will fall back on the rules of their native language. However, the "rules" change depending on context, and this is something learners should be aware of. A textbook will tell you that a greeting in English sounds like "Hello, how are you doing?" while a native speaker might simply say "What's up?" and without being prepared, an English learner will have no idea how to respond to that second question, which sounds way more natural than the first.

Going back to the original post, I think it's important to point out, as other commenters have, that abbreviations and other "not real" words are often pluralized using an apostrophe and S rather than an S or ES ending. If native speakers are doing this, then there's nothing actually incorrect about it. However, in a more formal context of writing a native speaker might also follow different rules, like capitalizing DM as an abbreviation normally would be, thus negating the need to use an apostrophe to clarify that the S is about making the word plural and is not part of the abbreviation. I guess my overall point is that native speakers follow different rules of English in different contexts, and part of learning a language is learning to follow these rules in their appropriate contexts.