Ok so this is an alternative to tweeting. What is the open source alternative to private fb groups and fb events?

EO: "the immutable biological reality of sex"

Biological reality:

Ever join a new social media site and post literally nothing and all of the sudden a bunch of your followers are random dudes you don’t know in real life (I see you- 7 other people I actually know). #firstpost #stopfollowingme