I've had success building relationships with individual Mastodon users, interacting and commenting on each other's posts for a while, becoming mutuals, then sending a DM along the lines of
"Hey! I noticed we follow each other, and I'd like to get to know you better! What are your interests?"
and building from there. The conversation usually moves to a chat platform fairly quickly (if it manages to continue) and I've started a friendship or two that way.
Meeting people in Matrix spaces is still my go-to, though.