macosnotificationswidgets
Half of my Notification Center widgets (most importantly, Weather) suddenly disappeared, and can no longer be added back by clicking Edit at the bottom of the Notification Center. Those that remain, like Today and Tomorrow, show no information.
They are also missing from → System Preferences → Extensions → Today.
I’ve tried reinstalling the macOS 10.12.3 Combo Update, with no success through multiple reboots.
The Facebook integration wasn't available when this question was asked - it was added in OS X 10.8.2 in September 2012.
To activate Twitter and Facebook sharing, you must first configure at least one Twitter and / or Facebook account via System Preferences -> Mail, Contacts & Calendars -> Add Account -> Twitter (or Facebook). Once you've successfully added a Twitter account, the Twitter widget should appear at the top of Notification Center.
System Prefs > Desktop & Screensaver > Desktop can rotate pictures from any specified folder, natively - no widget required.
Tracking your IP can be done with many tools - this one from dynDNS might do the trick [not something I've ever used as mine is static] Dyn Updater
Screen-fill options as below...
Best Answer
Apple has a knowledge base article for this bug occasionally affecting macOS Yosemite, El Capitan, and now Sierra. This happens most often after installing an OS update.
To reregister all Notification Center widgets and Share Extensions, start a Terminal session, and enter the following:
(Triple-click the above line and press ⌘ cmdc to copy, then ⌘ cmdv inside Terminal.)