Recently, I was out of town for about a week, and didn't check either Weasyl or FA for a little over 8 days. When I got back, I decided to run the math on the number of new submissions on each site.
tl;dr: FA gets about 2.6 times the number of submissions that Weasyl gets, at least for the set of critters I watch.
On FA, there were 176 new submissions. Of those, 10 were "ads" of some kind (YCH, commissions open, now streaming, etc). Taking those out, there were a total of 166 new submissions.
I am currently watching 152 critters on FA.
On Weasyl, there were 66 new submissions. One was a YCH ad, so taking that out, there were a total of 65 new submissions.
I am currently watching 118 critters on Weasyl.
I am watching some of the same critters on both Weasyl and FA. I haven't counted exactly, but I think it's at least a dozen critters. 11 of the new submissions were the same on both sites.
After taking out ads, FA had 2.55 times as many submissions as Weasyl.
Based on the number of critters I am watching at each place, and assuming that everyone posts everywhere at the same rate, I would expect to find about 1.3 times as many submissions on FA as on Weasyl. I found more than that, so I still think FA gets more traffic, but it's not as big a difference as I thought.
After I did it, I realized I should have noted how many views each new submission had, and compared them... but I didn't. :D Maybe next time.
I did this same experiment just about exactly 2 years ago, and at that time, FA had 2.3 times as many submissions as Weasyl.
I don't have data on any other furry art sites, or "general" art sites like DeviantArt, because I pretty much don't use them. These numbers are based on my experience alone - who I am watching and what I like - and may not reflect everyone's experience.
I mostly don't upload artwork; I just favorite and comment.
I am not associated with FA, Weasyl, or any other furry art site, as a staff member, moderator, programmer, or anything else.