User synchronization between sites in MS network.

I am running WordPress version 3.5.1 in a directory-based MultiSite Network with BuddyPress 1.7.1 and BBPress 2.3.1.

I'm finding that my users' information is not synchronizing between sites. For instance: a user logged into the main site and uploaded an avatar but that avatar did not show up on their account on a sub site and is not showing up in their blog posts on the sub site.

Am I missing something? I thought that all of the user accounts came from the same master database. Do I need some sort of synchronization plugin to keep my members in sync across the sites? And, if so, how is the activity stream handled? i.e. if a user posts a comment on one site will that comment show up in their stream if they later log into another site?

Please advise.