Snapchat
Snapchat Family Center
Snapchat is built around messages that disappear, which makes covert monitoring apps tempting. Family Center offers the honest alternative: your teen receives an invitation and must accept it to take part, and both of you are notified when that status changes.
Once connected, you can see their friends and how they may know them, who they are chatting with and their average time in the app, all without the content of private conversations. Location works as a two-way request, so you share yours in return. A new friend with no obvious connection to your teen's life is exactly the kind of thing to raise calmly over dinner.
More about Snapchat