You might want to communicate that the site has changed, or the navigation is simpler, or members can now create their own blog posts, or an array of tweaks and changes you have made to the platform.
The problem is this; members don't care about the platform.
They care if it's broken. They care if it's difficult to use. They care if they can't instantly find what they're looking for. They care if their private details are being mishandled.
But they really don't care about the platform. The platform should be transparent.
If you're going to communicate with members, it should only be really interesting things — otherwise members begin to ignore your message. Tell members what the latest popular discussions are, share amazing stories from members, let them know about exciting events that are coming up.