So you have a Facebook account, and you have a Playstation Network account, and you want to know how to link them. Thankfully, Sony has made it very intuitive, very easy, and very fast, which is the theme of the Playstation 4 all around this generation.
If you're setting up the PS4 for the first time, and you're setting up your Playstation Network account, it will give you the opportunity to put in your Facebook log-in and connect it that way. Beyond that though, it's very contextual, and that's sort of ties into the dynamic menu, and this idea of bringing up something when it's required as fast as possible. So if you're trying to share videos and photo through the share button, it's going to prompt you to put in your Facebook account information so that you can do that, and so that's a way you link it up that way.
So there's no, like, place in the settings menu where you can really just drop in and put in that info. It's gonna pop up when you need to, and once it's in, the two accounts are connected and you're good to go.