Flipboard is one of the biggest Fediverse platforms out there, second only to Threads I believe (unless you count WordPress, maybe?). 145 million active users. I think it’s mostly used by younger gen X/older millenials.
It didn’t have ActivityPub integration originally but promised to add it a while back, kind of like how Tumblr is joining the Fediverse, although Tumblr doestnd seem to implement ActivityPub just yet.