Tumblr is back on a roll.

After Yahoo spent $990 million to acquire the social blogging platform in May 2013, its traffic appeared to have reached a plateau, leading some critics to question Marissa Mayer's investment.

But new data from Global Web Index shows a revival in Tumblr's momentum. In the last six months, it experienced a remarkable 120 percent jump in active users, making it the fastest-growing social network -- topping YouTube, Pinterest, Twitter, and Instagram. Although the largest social service in the world, Facebook is now the slowest-growing of the bunch, with only 2 percent growth in active users.

According to the Global Web Index, an active user is someone who has used or contributed to the platform at least once in the last month.

So how did Tumblr manage to more than double its number of active users in just six months? Below are a three reasons that could be behind the recent surge in Tumblr's user engagement.

Embracing Pop Culture

In January, Tumblr hired its first head of media, Sima Sistani, who spearheaded partnerships with popular TV shows like The Voice and The Walking Dead. It also extended its pop culture appeal with live events that featured prominent newsmakers and entertainers, from President Obama to Outkast. Taylor Swift became active on Tumblr this year, joining other such big stars as Britney Spears and Lorde, who routinely post personal updates on Tumblr and reblog videos, photos, gifs, messages, and art from fans.

Designing a Beautiful Mobile App

With all of its competitors doing most of their business on mobile now, Tumblr has been racing to catch up. In May it launched a feature that allows users to customize how their blogs look on the mobile app. To prevent major eyesores (remember MySpace?) the customization options are limited to keep with the general architecture of the Tumblr app. Throughout the year, Tumblr also released multiple updates on both iOS and Android to keep up with new operating systems. Today 65 percent of the platform's active users use the mobile app, according to Tumblr, up from 50 percent back in January.

Releasing Features Nonstop

The mobile app design was not the only thing the Tumblr team kept updating. This year saw Tumblr integrate with Vine, update its video player, incorporate tagging other blogs in posts, and make other minor tweaks to browse, search and display capabilities. Overall, it was nonstop iteration and feature releases, which little by little has made Tumblr better for user engagement.