As a card-carrying linguist (even though they don’t pay me to be one), I am of course honour-bound to repudiate any claims of better or worse grammar. There is just more formal and less formal grammar, and you use the appropriate register and grammar in the appropriate circumstances. And “proper” grammar is quite improper in informal situations. Try speaking the Queen’s English on the factory floor. (Or in America.)
That said, what is going on is part of a more general devaluing of formality in Western society: it is seen (not unreasonably) as bound up with hierarchy and insincerity. Who wears a suit and tie to a classical concert any more? They did 30 years ago.
Formal grammar has its place, but social media is not it. Informal grammar, conversely, is seen as intimate, hip, and/or playful, which is an asset in social media. Which is why people on social media can go out of their way to ignore formal grammar rules.
They’re not ungrammatical, as far as linguistics is concerned: they’re not jumbling words in random order. They’re just following grammatical norms outside of formal written English. Abbreviations and creative spelling are where the more overt rule breaking lie.