I think it is probably simply people are not on, or simply not looking.

But another possibility could be a shift in demographics, or also the content in the pictures. (for example, If my characters are not wearing diapers, I will not get pageviews for the most part).

As for people leaving, people have been leaving FA due to changes in some of their rules (the new guidelines were confusing to many as best), as for the actual number whom left, I am afraid I do not have the answer.

In my personal opinion though: I think popularity in this sense is overrated anyway.

Yes, you're probably right.