I was on Identi.ca back when it first launched and I joined Mastodon a few years ago. Identi.ca had a decent number of FLOSS devs at the time, but by the time of Mastodon, Twitter was ascendant. So “no one” was on Mastodon. Even a few of the FLOSS developers I followed on Mastodon never posted on there. Network effects - it’s the reason almost everyone who threatens to leave Facebook never does; social media is only useful if you can be social (ie your friends/acquaintances are on it).