With over 1 billion MAUs, you may think that Facebook would be the most popular social site among those aged 13-25. But you would be wrong, at least according to a new survey.

Y Combinator partner and Posterous co-founder Garry Tan, with the help of Survata, looked at the social networking habits of teenagers and young adults. What he found was that Facebook is not the most popular of the bunch.

That honor belongs to Tumblr. The survey broke respondents up into two groups – those aged 13 to 18 and those aged 19-25. Both groups reported higher use on Tumblr than on Facebook. Combined, 54% of the 1,038 respondents said they used Facebook regularly. The survey defined regularly as “several hours a week or more.” But edging Facebook out by a small margin was Tumblr, which registered 59% use with those aged 13 to 25.

Twitter came in third of the social networks studied, with 20% use across the entire sample. Instagram logged 16% use and newcomer Snapchat saw 9%.

They also found something interesting – the younger the respondent, the more social network-happy they were in general

“Survata cofounder Aaron Wenger noted: ‘Usage levels were higher among 13-18yr olds than 19-25yr olds for every social network. The biggest differences were for Snapchat (13% for 13-18yr olds vs 4% for 19-25yr olds) and Instagram (21% vs 11%).'”

Still, Facebook is still incredibly popular among this age group, but it’s interesting to see that the younger crowd is not completely loyal to big blue.

[Garry Tan via Business Insider]