I think you're confusing the data behind the FA graph, which measures "online users" - that is, those active within the last hour (or possibly less; they don't say), and so comparing two different things - Weasyl's daily users with FA's peak active users.
It would be reasonable to assume that FA has a similar pattern of use. If so, then we need to compare peak usage. On Weasyl it is 800 registered users, while on FA it is 9000. This suggests that FA is roughly 11 times busier member-wise than Weasyl, at around 55,000 registered users in a day. (FA also appears to have a higher proportion of guest traffic, even accounting for the graph scaling and y-position.)
Inkbunny doesn't provide graphs, but the
stats page shows over 15,000 active members per day (peak hourly users goes up to around 1,700-1,800). SoFurry does not provide activity statistics, but third-party traffic graphs show it roughly as active as Inkbunny. Neither is close to FA's traffic level yet (e621 is
far closer, but a different kind of site), and Weasyl is about two to three times less active than IB or SF.
However, the fandom is large, and all sites are "competitive" in the sense that each has members who are choosing to use it as their primary gallery. Inkbunny was at Weasyl's traffic level a few years ago, so they just have to build on what they have. An uncompetitive site would be the VCL, which
retains just a few active users despite its
submission base.