The purchase of Tumblr was supposed to expand Yahoo's audience in a major way, but there's a chance that the blogging network may not be performing as well as it was expected to. Traffic to Tumblr's desktop and mobile websites appears to be on a drastic decline this year, leaving its websites with fewer views than they've seen since 2011, according to measurements by Quantcast. While unofficial traffic numbers can be unreliable, Tumblr has actually enabled Quantcast to gather data directly from its website, which suggests that its measurements have a good degree of accuracy — though notably they don't include mobile apps.

At the beginning of 2013, Quantcast says that Tumblr's monthly page views were in the high 16 billions. As of last month, that number has dropped by about 4.7 billion views. While 16 billion was certainly a peak for the network, Tumblr's numbers consistently hung around 15 billion throughout 2012.

While that seemingly signals a major decline in Tumblr's traffic, there's a good chance that something else is at play: Tumblr's mobile app. Tumblr aggressively promotes its app to users who visit on the mobile web, even going so far as to display a giant prompt at the top of user's dashboards saying that the app is "faster and a zillion times better" than the web. As it is, the blogging service is geared toward a younger demographic of users — exactly the kind of people who are likely to shift their habits to mobile devices. And according to Yahoo, that's happening in a big way. As of May, the company reported that over half of Tumblr users were taking to its mobile apps each day, with an average of seven daily sessions each.

In a statement to BuzzFeed, which originally noticed the trend, Tumblr pointed out that Quantcast also couldn't measure new pages that weren't explicitly loaded by the user, such as when a blog is set to infinitely scroll. Because of those hidden metrics, Tumblr could still be growing — but seemingly in a different way than before, and one that both Tumblr and Yahoo are going to have to work toward strongly monetizing.