Data collection, anti-trust and privacy policies. Right now it is the wild west and consumers have no idea what is being collected and how they are being profiled for ads. https://www.youtube.com/watch?v=wqn3gR1WTcA

It is pretty clear what is collected: https://www.facebook.com/policy.php

They also share what they know about you (used for ad purposes) and you can control (delete) that data: https://www.facebook.com/about/basics

I think it's pretty reasonable to argue that they're effectively monopolies in certain market segments and also that their businesses are causing extreme societal externalities that wouldn't exist if they didn't exist.

I don't think Facebook or Instagram is a monopoly in any sense of the word. Lots of people happily don't use either and there's a boatload of competing social media platforms.

If your goal is to thwart software monopolies (a pretty low impact use of FTC time imo) Google search or Visa & Mastercard (duopoly) should be a much higher priority.

The social externalities piece gets into philosophical territory. We "protect" people from cigarettes by taxing them heavily. Yet lots of people still choose to use them. Some would argue we should do the same for social media, others would argue that individuals should be free to decide for themselves. Still others would argue that comparing the health impact of cigarettes to that of social media is ridiculous.

IMO disincentivizing carbon emissions is a much more clear-cut externality to fight against!