I don't know about "MOST" as there are still a good amount of mainstream companies that advertise on the site. At least you wouldn't know it if you are still on the site constantly. And I'm mostly going on about the users there who don't like the changes there and constantly complain about them in articles or youtube, yet it is the only way they insist on engaging with their followers still.
The thing about Mastodon and Lemmy to users who exclusively use those sites is they are themselves a bubble. There are weeks I spend most of my web activity on kbin, and would believe reddit is on its death spiral. Yet I peak over there, and it's about as active as when I left it. I even see articles about reddit or twitters activity recovering that I never see get shared here, because at the end of the day, the narrative we want to hear is "Corporate site is dying, we are the future!"