Learn Finnish: Keskustelemme sosiaalisesta mediasta.

The verb keskustella (to discuss) requires the topic you are talking about to be in the elative case. While in English we say "discuss something" as a direct object, Finnish verbs of speaking and thinking often frame the topic as something you speak "out of" or "about," using the -sta/-stä ending.

How exactly does the adjective sosiaalinen become sosiaalisesta?

Finnish words ending in -nen change that ending to -s- to form their stem before adding most case endings. So, the stem of sosiaalinen is sosiaalis-. Because the noun mediasta is in the elative case, the adjective must match it, so we add -esta to get sosiaalisesta.

Why do we add the ending to both words to make sosiaalisesta mediasta?

This is called adjective agreement. In Finnish, an adjective almost always has to match the noun it modifies in both number (singular or plural) and case. Since the verb requires the elative case for the noun media, the adjective sosiaalinen gets pulled into the elative case too.

Is media treated like a normal Finnish word when adding endings?

Yes. Because media is a relatively modern loanword ending in a vowel, its stem is exactly the same as its basic dictionary form. You simply attach the -sta ending directly to the word without making any stem changes.