Moderators can pin posts, leave them at the top of the stream and make sure the message gets to anyone visiting the community stream.
When you click on the little triangle to see the post options, you can see who pinned the post and when.
Group Pinned Posts
Groups also got their own pinned posts. Right now you can pin posts within a group. Yes. Multiple posts within a group can be pinned. If you are a member of a group and are following it you will see the pinned post on the main activity stream as well. So no worries, group owners, managers and site admins can pin! Pin! Pin!