According to the raw data download, there are 365 data points, but 104 answered "I don't use any of these" which are stated to be excluded, so the actual amount of teenage respondents is 261?
One school may be very facebook heavy while another may mainly use twitter.
Hope that helps.
I suggest reading "How to Lie with Statistics"[1]
[1] http://www.amazon.com/How-Lie-Statistics-Darrell-Huff/dp/039...
Edit: I guess that sounded a little harsh. No doubt, Snapchat's growth has been explosive and if this was just another blog-spam post about that fact, I'd let it be. But you're selling a statistical software! I'd expect better from people who know what they're doing. Also, "How to Lie with Statistics" is a really good book, should be required reading for anyone dealing with numbers. Did not mean that as an attack on your product.
Most people are surprised how "few" respondents it takes to get to 5% margin of error at 95% confidence levels: https://en.wikipedia.org/wiki/Margin_of_error#Different_conf...
In this case, .98/sqrt(365) = 5.1%
So, perhaps counterintuitively, 300-400 respondents gives good read on large populations (like Snapchat users...or the US population!).