Desktop notifications on MacOS for Facebook and Twitter will be history, at least for a while.
Starting with iOS 11, Apple withdrew its iPhone and iPad operating system for Facebook, Twitter, Flickr and Vimeo. The same thing has done recently on the MacOS Mojave beta, and most likely this integration of third-party internet accounts will also be missing on the final version.
In the panel Internet Accounts, up to MacOS Hight Sierra there were separate login options for Facebook, Twitter, Flikr and Vimeo. With the withdrawal of these accounts, users are unlikely to be able to receive notifications via the "Notification Center". Most likely Facebook will have to offer a solution in the future.
Most likely, these options were eliminated by Apple's desire to store as few private accounts as MacOS 11.14 and iOS 12.
Support for third-party e-mail accounts is maintained. iCloud, Exchange, Google, Yahoo! and AOL.Facebook & Twitter Retrieved from Internet Accounts on MacOS Mojave