Q: Share menu - third party extensions missing

Hi

Since installing El Capitan some of the options in the share menu have disappeared, e.g. Facebook, Twitter, Mail. They're also no longer listed in my Extensions Preferences. I go to Internet Accounts and click the Share Menu box for both Twitter and Facebook, and a blue tick appears but then immediately disappears... thus I assume not linking either account to Safari.

Why is this happening? Can anyone help?