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

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!