I'm not good at the popularity thing, but I know that a lot of people in the last few years moved from DA to Tumblr for a few reasons.
1- The rules on DA favour some nudity over others,
2- The amount of comments people were getting were less
3- It was easier to find your fandoms on Tumblr
4- Cause the site coding is becoming a screwed up mess of accessories and spam.
Obviously PD doesn't have an issue for the first one. PD has a really cool and simple policy on nudity which is really refreshing.
The amount of comments I got I noticed dropped significantly after groups were introduced to DA, I assume cause of spam. People would submit a picture to 20 groups to get noticed, but what ends up happening is that the average user actually shuts off cause their inbox is getting 1000 deviations in an hour. Many groups only allow a submission a week or day, and it still gets slammed. I know if a group has over 1000 deviations in it, I don't even bother looking through it.
With Tumblr, I can't explain why people moved to there of all places. I think they'd move to Pixv is it were in English? I think that the interface makes the user feel like they have more control over how they browse though. Specially with the Xkit extension. You can make a side blog of a single thing you have interest in and people that are interested can follow and interact with 1 to 1 just that and it remains separate to your other stuff and not associated to your main blog unless you want it to be, blacklist words that trigger bad memories, or ignore blogs that post content that doesn't interest you, liking things is a very easy to find button, you can show people that follow you stuff that you like and you can spread the word super fast of cool things. I don't know how that feel can be applied to an art community.... I think though that the freedom that Tumblr has is a big appeal to it's following. People feel free to talk about relevant topics and feel free to post what ever they want cause the people following you theoretically want to see it, and if they don't, they just unfollow, and finding new blogs is organic, cause you find them by having a popular post that gets shared.
I don't know if any of this helps. One could say that people on the net just need to calm down cause it's just art, but there's also a lot of sense in making people feel like they have a personal experience. A lot of people are starting to want to have an anon type persona that they can go wild with, and seeing other things that person liked can help you find things that you might like too. Deviantart has art roulettes on the side of the page of stuff in their gallery / stuff similar / other junk they've since added that has again cluttered it too much. Whether it's worth introducing though is another thing.
I wonder too sometimes if art communities aren't entirely what people want anymore. I mean, it's efficient for art, but people are commenting less on art now, and sharing it more, interacting with the artist outside of the art via chat. They want others to see art/ists they love, but they don't nessecarily have the words to convey thier love on each piece of art.
Sorry- this is something I've thought a lot about, and perhaps I'm over thinking it based on what I see, and not the reality of what people want. I hope what I mean came through in my garble?