As a person close to the "teen" demographic, I can tell you that teens still use Facebook. Thinking of social media as a zero sum game is a common mistake; each has its own use case. For instance, Facebook is usually used when chatting on desktop/laptop with friends, adding new people you met, or chatting with people who you aren't really close to. Also, Facebook is widely used for groups and events, which no other social media provides. Snapchat might have replaced some aspect of the Facebook newsfeed but people still post things on Facebook. When a teen says, "omg Facebook is like so old and no one ever uses it". They mean to say that they don't use it as much but they still use it for things I mentioned.

Edit: It is important to note that Facebook has the "identity" aspect down like no other.