Tumblr Tops 13 MM U.S. Uniques in July

Tumblr, the simple sharing service and blog alternative, continues to attract a record number of visitors each month.

According to comScore, Tumblr scored 13.4 million unique visitors in the U.S. in July — up 218% from the same time last year.

The blog-meets-social-network service has seen its most explosive growth in the past few months, according to comScore’s data, upping its unique visitor count by more than 5 million from April to July.

comScore attributes the pop to Tumblr’s “network effect.”

“The network effect is predicated on the idea that the more users that are part of the system, the more valuable the system becomes to users, which creates a virtuous cycle that pulls more users into the system and gives existing users more incentive to participate,” comScore’s Andrew Lipsman says. “This concept is an important reason why we often see that once social networks achieve critical mass, the network effect takes hold and adoption tends to accelerate.

Founded in 2007, Tumblr surpassed 20 million blogs in June of this year. The blog figure is now fast-approaching 28 million.

It should come as little surprise then that Tumblr is rumored to be soliciting investors to participate in a $75 million to $100 million round of funding that would put its valuation near the $1 billion mark — $800 million, by The Wall Street Journal’s count.