It is clear at this point that Facebook has a monopoly on online human-to-human interaction that no private forces, market or otherwise, will break in the foreseeable future. The network effects from a billion users are unsurmountably large. If we take Metcalfe's Law literally, even a social network that accumulates a hundred million users will be a hundred times less powerful than Facebook. In fact, you're probably confused by the title of this post: What Facebook alternatives?
Facebook is furthermore unlike the other American technology giants in that it alone locks up all its users' interactions inside its walled garden. Apple, Alphabet, Amazon, and Microsoft are, to greater or lesser degree, porous at the edges — you can use an iPhone to chat with people who don't have iPhones; you can use Gmail to email people who don't have Gmail; buying things from Amazon doesn't prevent you from buying other stuff elsewhere; even Microsoft has realized belatedly that it is not the center of the universe & its products have started playing nice with others. But Facebook locks up your posts, locks up your photos, locks up your entire social identity inside its prison. There simply is no way to interact with Facebook users except by creating a Facebook account yourself and creating content that further entrenches Facebook's monopoly.
what'd help community groups get off of facebook? I see a lot of small community groups trying to plan events via facebook.— alienghic (@alienghic) September 20, 2017
In my experience, there is no plausible alternative to Facebook if you're working with ordinary people. https://t.co/B5gLjuERzc— Zeynep Tufekci (@zeynep) September 20, 2017
The gradual decay of open Internet protocols as human interaction disappears down the black hole of Facebook's ever-expanding digestive tract has been one of the great disappointments of my lifetime. In the end, AOL seems to have beaten the Internet after all.
I have opted for only de minimis engagement with Facebook, and more or less refuse to communicate via its platform. This has probably attenuated some of my relationships with people in a regrettable way (if you're somehow reading this and you wish this hadn't happened between us, send me email! it still works!) but the actions of conscientious objectors like me have not made the tiniest scratch on Facebook's dominance.
It is only a matter of time before governments realize that this entity must be regulated, whether under antitrust law or otherwise. The question is what will happen then.
In my opinion, it is clear what the ideal outcome would be: forcing Facebook to adopt open APIs that give users transparency, portability, and interoperability. Users should be able to see the data that Facebook has stored about them. Users should be able to export that data in toto to competing platforms. And users should be able to interoperate between Facebook and other social networks — a future version of Diaspora*, for example, should be able to see and interact with Facebook content generated by that Diaspora* user's social network, and vice versa; interactions between users across platforms should be reflected accurately on both sides. A user would thus be able to leave Facebook without severing their ties to the users they have left behind.
In a world where these APIs existed, users would have a way to reject Facebook's toxic business model and questionable privacy practices without exiling themselves from their social life. In Hirschmanian terms, users would have the option of exit, not just voice, as a way of signaling dissatisfaction. Facebook would probably even get healthier, as a product, as a result of the opportunity for meaningful competition.
This outcome is exceptionally unlikely. Government regulation of Facebook, although likely inevitable, is also likely to be ham-fisted and ineffective, simply because governments are terrible at understanding technology and rarely have the political will to impose effective solutions even if they knew of them. The last time the U.S. government, for example, used antitrust law against a technology monopolist, it basically bogged down the company in red tape for a decade but did little to meaningfully give its competitors an opening in the allegedly monopolized market. Windows is still by far the most widely used desktop operating system and the web browser that finally dethroned Internet Explorer on Windows did so through incredibly aggressive Internet marketing, not by using the remedies forced on Microsoft by antitrust law.
However, there is one thing that technologists might be able to do to make the desirable outcome marginally more likely, and that is to develop the protocols, and plausible implementations thereof, that would allow effective federated social networking to be mandated by government decree. Diaspora* may have made a significant dent in a subset of the technical problems, but there are significant open challenges inherent to federated social networking that I suspect have not been solved.
Critics of Diaspora*, Mastodon, etc. thus misstep when they observe that organic growth of these platforms is limited. The ultimate destiny of a successful federated social networking protocol, if one ever arises, will be to stock the toolkit of a future regulator, not to overtake Facebook via organic growth.
There is enormous inertia—a tyranny of the status quo—in private and especially governmental arrangements. Only a crisis—actual or perceived—produces real change. When that crisis occurs, the actions that are taken depend on the ideas that are lying around. That, I believe, is our basic function: to develop alternatives to existing policies, to keep them alive and available until the politically impossible becomes politically inevitable.
 100M is, to an order-of-magnitude approximation, the size of Snapchat's active user population. Observers who think Snapchat is a credible challenger to Facebook are off by a factor of a hundred, not a factor of ten.