For Ross, it would all be about Facebook. He'd be that annoying friend you have, posting motivational quotes, The Big Bang Theory GIFs, and (of course) dinosaur-related content. Oh, and he likes Minions. We bet he does.
And finally, Monica. She'd be the queen of Pinterest, we're sure of that. There's no other social media platform that could cater so well to her obsession with weddings, cleaning, and food - all at the same time.