Has everyone forgotten about phpbb? (Or whatever the slightly more modern forum software is…) Historically forums are the most successful small social networks.
Not sure what your experience is, for me forums still work great but they have transitioned from general social hangouts to very specialized niche communities. I don't mind that and still get a lot of value from e.g. music production forums like Mod Wiggler and Gearspace. However, other than HN I don't frequent any other small-ish communities where conversations can go outside of the stated purpose.