> ... persistent group chat, shared chat history with a search function, synchronized unread/read statuses....
Across all my devices - Web, Linux, Android and FirefoxOS. No advertising.
Telegram - http://telegram.org
I've been using it for several months now and am very happy. Would love integrated voice calling, especially to landlines/mobiles, and hope someone develops a plugin for this soon (perhaps the guys at Jaconda[0] will do it).
I have just one regular contact still using Facebook messenger, and our conversations are now very disjointed, as sometimes I don't login to that service for several days at a time. I don't use Skype as a messenger service, but do still have about four or five contacts who are wedded to it for free voice calls. I use WebCallDirect[1] for cheap calls to landlines/mobiles.