With the demise of G+ a lot of community owners are suddenly looking for a new home. Since a lot of G+ users despise FaceBook (and rightfully so) other social networks are mentioned. The problem with a lot of these networks is that they are centralized and proprietary making the users depending on the whims of the owner of the network. Below I summerize the FLOSS options for people interested in setting up a community. This is not an exhaustive list. I encountered these options after discussions often on G+ and I experimented with some of these options during the last couple of months.
Self-hosted or not
An important decisions is whether or not to host the forum or community yourself. If you want to host it yourself you'll keep full control of the server however the maintenance is considerably more than with a non-selfhosted solution. You also need to install the software on a server and configure it.
Both are macroblogging social media networks that offer the possibility to create a forum. I found the Osada forum particularly easy to set-up and maintain. The UI of both doesn't look very modern but the functionality needed to use and maintain a forum is all there.
Movim (self hosting possible)
Movim is social platform that let you share and chat. But it's also possible once you've created an account to create a community. The UI looks modern but some forum admin features are missing (or I couldn't find them). As an example as an owner I couldn't ban someone from the community.
Flarum (only self hosting)
Open source forum software that is currently in beta. Nice, modern UI. I've read some concerns about the beta status and the stability of Flarum.
NodeBB (self hosting possible)
Open source forum software with a modern UI. You can either self host for free or use a NodeBB hosting plan that comes with a price tag.