Having a low active user count doesn’t mean there’s no one to interact with. I use it sometimes, I see lots of interaction. It’s not a success as a business (which it’s not necessarily trying to be) but that doesn’t mean it’s not a thing (a protocol) that many people are regularly using.

>This is enough to tell us that Mastodon is indeed a failure and has not replaced Twitter after 6 years of marketing itself as a Twitter alternative.

I think this is pretty obviously false. There are vibrant communities on Mastodon that left Twitter so they could exist in peace. Furry and furry friendly community, lgbtq+ webcomic artists, sex workers are a few off the top of my head that created their own communities.

And I would have thought this was self evident, but you can be an alternative and succeed as an alternative without it meaning you have to replace the original or absorb all the active users of the original.

Despite the name of your source, it seems to consider less than half of existing Mastodon servers. According to my own crawler, the up-to-date active user count is 540,258.