Facebook is about to go live with a feature that will put together a video of you and a friend's time together, as a way to "say thanks". It'll go live on phones and desktops at the same time, with a video recorder and all of your old media to deal with.
Right now, the official announcement page links to another Say Thanks page that doesn't seem to work. I'm guessing that will be fixed up soon, so in the meantime you could say thanks the old fashioned way — by saying "thanks". Or sending a fruit basket.
This seems like a feature that will only really work if you have lots of pictures, videos, and tagged posts with your friend, as material for Facebook to work with. Otherwise it'll be a pretty short thanks. For Facebook, it's probably another in a long future line of attempts to make old content new again.
But the feature should be live soon, so go nuts. When saying thanks is just a matter of pushing a button, why not? Sentiment without effort!