On Thursday a new study from Piper Jaffray suggested that Facebook Inc (NASDAQ:FB) was no longer the most popular social network among teenagers. Citi analysts released a report countering that, saying that investors don’t have to worry about “younger audiences” abandoning Facebook. However, their report focuses on the 18 to 25 demographic rather than teenagers. This is an important distinction to make when weighing their comments against Piper Jaffray’s findings.

The problem with anecdotes about Facebook

Analysts Mark May, Nathaniel Brogadir and Kevin Allen say most of the claims that “younger audiences” are abandoning Facebook are only based on anecdotes. They said most of the data doesn’t support these anecdotes, and besides, adults control most of the money right now, so maybe teenagers don’t matter as much. However, teens are the future spenders, so remaining popular among them is still a concern for Facebook Inc (NASDAQ:FB).

But the good news for Facebook is that it owns Instagram, which appears to be gaining in popularity among the 18 to 24 demo according to comScore and also among teens, according to Piper Jaffray.  comScore found that the demo makes up 21% of the time spent on Facebook but 54% of the time on Instagram. Across all of Facebook, including Instagram, the demo makes up 24% of all time spent there. Also Citi analysts note that the declines in usage of Facebook.com in the 18 to 24 demo are being offset by gains in Instagram.

I do see one problem with Citi / comScore’s numbers here, however, because the only age groups named here are ages 18 to 24 and 25 and up. There is no measurement for teens under the age of 18, so these numbers are assuming younger teens aren’t even using Facebook, which is clearly not right.

Facebook instagram

In terms of users, 30% of Instagram’s users are in the 18 to 24 demo, while 19% of Facebook.com’s users are in that demo. Across all of Facebook’s offerings, 22% of them are in the demo. Once again, these numbers don’t appear to assume teens between the ages of 13 and 17 are using Facebook.

Looking at trends rather than one month of Facebook usage

The Citi analysts note that usage trends differ greatly from month to month, so it’s important to look at longer trends rather than monthly ones. They note that in the demo they’re examining, total unique visitors to Facebook mobile fell 4% year over year in August. However, that decline came after larger gains in the two previous months, so overall, total time spent by the age group on Facebook Mobile is still growing.

Facebook mobile

And even though the number of unique visitors in the 18 to 24 age group changes month to month, the overall trend of average time spent by the group is still climbing significantly.

Facebook