Snapchat vs Facebook: Can the underdog get big enough, fast enough?

Facebook casts a long shadow. 
Now worth more than $340 billion, the social network lords over the biggest platform in the history of mankind. The company has either bought up its direct competitors or relegated them to also-rans. Few companies outside of Google, Amazon or Apple can consider themselves serious rivals to Facebook.
Snapchat wants to be in that discussion.  
The little app that could is now preparing what will be one of the largest U.S. tech IPOs ever, and the biggest sin