How exactly "do* you break up a Google or a FB so that (only one possible reason, but the one cited here) they don't control too much media/mind share?^
Laws usually want to be general, and my suggestion doesn't necessarily lend to that, but I'll suggest it anyway.
Facebook doesn't need to be broken up into several companies. It can just be shut down.
I don't mean that it should (justice-wise) be shut down. I just mean that we won't lack for social media. We will have social media alternatives the day after FB shuts down. Theres a chance we'll get something more open instead. There's a good chance we'll get several small replacements instead. There is 0-chance that we'll lack for ways to share posts and post pictures. This isn't Bell, where we need to keep the phones working. The phones will work fine with or without Facebook.
There is no need for a company to generate $70bn in revenue, in order for us to have social media. That's a key difference from all antitrust cases of the past.
YouTube is another sort of example. If it shuts down, alternatives will pop up with immediately...maybe open ones.
There are justice questions (is it fair to shareholders/employees/zuck?) There are legal validity questions (why FB and not apple?). But, for the practical questions... the problem is an easy one.
^fwiw, I also think this is the most worrying part. These companies have a tremendous control about how and what people think. They make Murdoch media look quaint.