I have a theory: the turning point in any social media service is when it moves away from having a chronological timeline. That's the shark-jump.
When you go from displaying content that the user largely seeks out to actively "curating" or promoting content, that's a big step. It's almost always driven by a desire to control what users see.
FB/Twitter/Insta have all followed the pattern—they bait-and-switch with a chrono timeline, then mix in the "secret sauce". Every time.
"I appreciate SDF but it's a general-purpose server and the name doesn't make it obvious that it's about art." - Eugen Rochko