One analogy/model I like is the gravitational effect of groups. This doesn't say much about their quality, but affects size.

People talk about the "value" of a network, but it can also be seen as power. A large network exerts a power, not only on those inside it, but also on those outside.

"Gravitational" large groups suck-in individuals, and other smaller groups, in a runaway agglomerative process, like stars. Once commercialised this happens by acquisitions and mergers.

Groups start out with control over their boundaries and size. You can take or leave them. Past some point they lose that. Other groups die because of them, and we get a "Monopoly" problem.

Facebook and Google were both harmless until it got to where you constantly heard about them from everyone you met. Then comes pressure from others to join. Eventually the cool, or individually empowering thing to do is to not be in that group. That can induce loyal stalwarts to leave too.

To press the "star lifecycle" analogy (perhaps too far), I think the jury is out on the end-life. Some may become Red Giants, just growing, diffusing into space but growing cold. To me, Google has become that, "ubiquitous yet amorphous" offering an extremely low value prospect but permeating too much. Some, perhaps like Twitter, will go White Dwarf, pulling-in to a smaller, super intense thing that goes bang! Although it has millions of participants, people outside it's small gravitational range are barely aware it exists.