Yea, teens don't do fb. It's an old peeps thing.
My older teens (now 16 and 18) have instagram. I also started an account when they opened theirs so that I could become more familiar with the app and also follow them.
It's diferent than facebook in the fact that people don't necessarily have to use their real names. so when you look at her friends list you will likely have to ask her who the people are. Also almost every teen creates a "regular account" and a spam account. Many teens have multiple accounts. they'll tend to post their safe stuff on the regular account and have spam accounts for multiple other reasons. Sometimes it's benign and just fun. My dd and her cousin have a spam account together that they just post silly pictures for example. sometimes kids will use that spam account to post more racy things on those.
You can also get more ads and see things that you didn't necessarily follow than you do on facebook.
Insta does seem to be one of the main ways teens communicate. That and snapchat which I totally agree with your discomfort with snapchat at her age.
I'd say this is a good first step. Make sure you have her password and she knows that you can login to monitor should you need to. make sure your boundaries for her in its use are clear.