It gets you solid 3-7 years of billions of eyeballs to monetize from, and nobody is better at monetizing than Google or Facebook.
There are 2 reasons why GOOG/FB are interested in picking Snapchat up. #1 user base #2 traffic. No other reasons.
#1. When FB strategist comes to Zuckerberg, he tells him: "listen this company is making dent in our traffic. If its continue the kids that abandon FB for Snaptchat will create $XX billions (sum A) of a hole in ads sales. So you need to go out and offer X% of sum A and pray they accept it.
#2. When FB ads vp come to Zuckerberg, he tells him: "listen, here are my ideas how we can serve ads to those billions of eyeballs. This will create $XX billions (sum B) of profits for Facebook. So you need to go out and offer X% of sum B and pray they accept it.
Thats it.
As of whether its a "startup" or not, I say it is exactly for the reasons other says its not: revenue (or lack of it). I don't think you can call a company "startup" only when they have a business model and or revenues. Snapchat through millions of uniques proved to the world they do solve some kind of problem, even if its as silly as flashing photos of a drunk teen for 5 seconds.
Edit: so the wisdom of the day would be: build a service for a huge user base that is important to facebook (youngsters), preferably on mobile (that is a future of computing/socializing).