I think my Mac swallowed Facebook and Twitter - How to get them back?
I’m sorry to say that since you’ve upgraded to macOS Mojave you’re sort of out of luck.

Apple removed the built-in macOS Facebook and Twitter integration in the name of security. 

Your main options are to manually share content via Twitter or Facebook directly, or with the use of browser extensions (although these won’t extend their functionality to Finder).

On the bright side, Twitter for Mac is coming back as a result of Apple’s Catalyst project, so we may see many other apps returning or launching on the Mac as a result. And that may mean sharing content will be easier then it is now, although not with the same integration within macOS we've seen in the past.