Approach it like evaluating dependencies, or maybe researching a political candidate. View a few different federated-social sites, with test accounts if needed. Rank what you like about them on a spreadsheet. Mark users or discussion threads you want to follow.
Then when you want to participate, that's your starting point. Lots of people are lurk-centric, but you are already in the comments section here, so moving your posting probably will not be the issue so much as contextualizing it, which the different software and instances do help with.
A lot of "community" is in having some ritual, some events to show up for, or some events you organize yourself. It doesn't take a lot of those to fill up a calendar, but it definitionally needs to be non-zero.