If you took a drink every time an economist used "network effects" to explain why Big Tech is so big, you'd get very, very drunk.
To be fair to economists, network effects *are* important to the Big Tech story.
A system is said to have network effects if it gets better when more people use it. That certainly describes Facebook - you join FB because of the friends that are already there, and then someone else joins because you're there.
https://en.wikipedia.org/wiki/Network_effect
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But network effects are how FB *gets* big, not how it *stays* big. Because even though you join FB to talk to your friends, the reason you stay there - despite surveillance and FB's many abusive tactics - is that leaving FB will cut you off from those friends.
There's no technical reason you couldn't stay in touch with FB friends without being an FB user. You can switch phone companies or email providers without walking away from the family, community and customers you're connected to.
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A "switching cost" is whatever you have to give up to switch between products or services - switching from Audible to a rival platform would cost you all your audiobooks, for example, thanks to Audible's DRM.
Facebook deliberately engineers its products to have high switching costs so that it can impose more pain on its users without losing them. So long as the pain of staying is less than the pain of leaving, Facebook calculates it can maintain its dominance.
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A conundrum for the fediverse is how to produce content that is engaging.
