I think my Mac swallowed Facebook and Twitter - How to get them back? I upgraded today from Sierra to Mojave and now my share menu seems to be broken.

Previously I could share content directly to sites like Facebook and Twitter. For instance I could right-click on a photo and choose to share it to Twitter, Facebook and so on like so:



Now if I click on a photo and choose to share, I don’t get Twitter or Facebook as options anymore:



How can I enable these options?
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.