Have they ever really had ideas in the first place? Content created by the users is the main reason why anybody goes to these sites, not because we're wowed by their brilliant ideas. r/place is apparently the peak of Reddit's fresh ideas and people are just using that to shit on Reddit's CEO, but again, r/place is nothing without an army of people contributing (or writing up bots that contribute). For the most part, people tend to hate whatever ideas that Social Media companies come up with outside of the core functions (seeing other people's content), look at how long old.reddit has hung around for because people absolutely could not stand Reddit's ideas. Every "idea" that Social Media companies come up with are usually weirdly stupid and have some sort of ulterior motive behind it (almost always profit/cost-reductions).

tl;dr Social Media companies aren't out of ideas, they just never had them in the first place.