The same will happen (and is happening!) to Instagram. They've recently changed Instagram to non-chronological "curated" feeds, and added a bunch of ads into the mix. What used to be a very pleasant social network has become way less enjoyable in recent months.
They also just (after years of resisting) opened up the posting API to some partners.
The same suckification cycle is happening (increasingly intrusive ads, algorithm tweaks, "here's a pic from someone you don't follow", etc.). The user decline happens later on in that cycle.
They also just (after years of resisting) opened up the posting API to some partners.