If I want to bring together a group of people, I have lots of options, from very structured - "I have a Facebook group for the sports team / political campaign I run and want to share information with players, get them to do things" - to the very ad hoc "I have a group of buddies who I get together with once in a while and to save time we've got a WhatsApp group." Both of these work well because they're linked with something I use all the time anyway - Facebook and WhatsApp. If I want to connect with random people on subjects I care out, there are subreddits.
The use case for this, on the other hand, is what? "I'm going to use a specific service that I'm going to have to persuade all of my friends to use, with no real added benefit?". I just don't get it.