Google+'s biggest rival (hint: it isn't Facebook)

It seems that everyone is quick to draw comparisons between Google+ and Facebook (or Twitter on occasion). Some have even said that the sole purpose of Google+ is to compete with Facebook. While that may be Google's ultimate goal, most people are overlooking the one social network that possibly has the most to lose: Tumblr.
Format
Let's begin with what is perhaps Tumblr's greatest strength: ease of use.  What Tumblr does best is allow its users to easily post videos, photos and short blog posts without having to worry about hosting, CSS or HTML. Google+ has not only duplicated Tiumblr's ease of use but has arguably Tumblr'd photo sharing funcionality by having the best photo sharing UI for any social network. Besides for the standard lightbox functionality, comments and mentions, Google+ also has a few features that appeal to the photo nerds out there (exif data being one of them). Trey Ratcliff of Stuck in Customs is a great example of how a photographer can use Google+ to showcase his phtoography.
Network
While there are many well established microblogging services out right now (Plurk, Jaiku, Identi.ca) that integrate blog posts, photos and video they lack one critical feature that has made Tumblr the powerhouse that it is today: networking.  Tumblr is currently the 10th largest social network with 6.8 million weekly visits. For years Tumblr was the perfect (and arguably only viable) medium for those who felt constrained by Twitter and Facebook's character limitations but longed for the conversation and engagement that social networking offers. For Tumblr the "reblog" button made that possible and helped many blogs become viral sensations. Google+'s share button offers similar functionality.  
How does Google+'s network compare to Tumblr's?.  It took Google+ less than a month to hit the 25 million user mark and according to some, it will reach 100 million users faster than any other social network in history.
So what does this all mean? Will Google+ lead to Tumblr's demise? Probaby not but for the first time in a long time, Tumblr has reason to be nervous.