You can post to a FB page using Publicize:
https://jetpack.me/support/publicize/#facebook
See the section that says this:
Note for connecting to pages: You will need to use a Facebook account that has a Profile with administrative rights to the Page. A Page-only account can not be used with Publicize.
But at the moment, there’s no connection to a Google+ page. Fingers crossed that will be available in the near future!
