Niche Socials Breaking the Feed

What are niche social apps, why have they become so popular, and what does it mean for us?
Niche social apps take one specific element, whether that's one subject (gaming or traveling), one audience (by age, socioeconomic status, or viewpoint), or one function or feature (share one photo per day or messaging only) and attempt to grow an audience that way.
We discuss the advantages and disadvantages for the apps themselves, but especially for the rest of us.