1. “Media” is a plural term.
2. The singular is “medium”.
3. “Mediums” should only be used as a term if you’re talking or writing about more than one psychic.
4. “Social media” is a plural term. Think of it in the same bracket as “social tools”, “social platforms” or “social channels” if it helps.
I know we don’t speak Latin anymore. I know that there comes a time when the language inexorably changes and you just have to go with the tide (check out the double possessive rule; if it’s been in operation for centuries, it’s time to give up the fight).
I just can’t help being driven slightly insane by the huge number of articles I read every week entitled “social media is [insert insightful comment / occasional platitude here]”.
I will now go back to being sweetness and light. I’m even planning some baking blogging* for you soon.
*While we’re on the subject of unbelievable pedantry… When you mean “I want to write a post about this”, try not to say “I want to write a blog about this”. You’re probably not writing an entire blog about that subject, just one post. I reckon we have probably embraced “to blog” as a verb, though, so you could try “I want to blog about this” as well.
MOKUSKA 
True and easy to forget. With the first quoted error, what they mean to say is something like this, “The phenomenon (or insert other term) of social media is …”.
Sometimes saying “social media are” isn’t right, as well, because we’re talking/writing about the idea of it, but not the outlets in fully plural terms.
Just like other things of the past, not the word media itself, but the phrase “social media” is (another example like above) becoming something new, entirely.
Makes English types squirm. It’s quite uncomfortable to be us … and sometimes we even loathe ourselves for forgetting as well and going along with the flow.
We’re human! There, I said it. Phew.