My understanding is the Facebook container add-on blocks phoning home by Facebook like and share buttons. You are right that ad networks can still share data (use uBlock Origin) or websites can share this in the back end (nothing we can do about I guess) but the Facebook container is not nothing.
The Facebook Container add-on is distinct from the Multi-Account Containers feature.
It's much friendlier than building your own containers, since most of the hard work and decisions have been made by the developers of the Facebook Container add-on.
You should definitely install the Facebook Container if you have a Facebook account, it traps that neatly inside a box. These days it can even be taught about any sites you use Facebook to authenticate with (this isn't a good idea but whatever) to bring them inside the container while everything else stays outside.
