Value props: can be any of the following
- small/closed groups (groupme, skype, what'sapp ...)
- interest-based/open groups (hackernews, reddit...)
- local groups (circle..)
What are some of the good ones to really learn from?
Another good one that I've actually used is Slack (https://slack.com/). You can invite people by email, or set it up so that anyone with an email address at a specific domain gets automatically added to your organization. So if you had a company called FooBar, with @foobar.com emails, you would get a sub-domain like "foobar.slack.com" and everyone with a verifiable @foobar.com email address will be added to that organization.
Then within slack, you see everyone logged in (from your team), can create public channels (it's made to look like IRC 3.0), and also create private channels. Images are displayed in-line, links to web-sites give you a preview or the page. It's a cross-platform browser-based system, with apps for Windows, Mac, Android and Apple iOS.
There's also an API, so you can have systems in your company posting to various channels (or you can connect up something like Hubot to do this). It's kind of nice seeing when a jenkins job runs, code is commited, or an issue is updated in jira.
Personally, I have been waiting for an open-source persistent chat server that is API-based, federation capable, and uses http(s) for transport. I'd like this system to have configurable levels of security (including unauthenticated/anonymous). Then there would be "adapters" made so that you could connect either XMPP clients or IRC clients into the system. Such a system would be fairly killer.