Crudely it can be summarised as a change from investors throwing money at tech in the hope of buying a share of future profits to wanting to see those profits. That means business models have to now actually deliver and a lot of then just don't - twitter, reddit, Gfycat - they're all part of the same phenomenon. Big revenues is meaningless without profit. While they may have value to the users, it seems the advertising funded models and data harvesting models just don't deliver profit for these kinds of services.
So they desperately try to adjust the models to maximise income and reduce cost. Enshittification ensues.