Movim is a decentralized social platform, written in PHP and HTML5 and based on XMPP. Database backend for caching. Seems suited for not-shared hosting because php-fpm is recommended, as well as Websockets and a long-running daemon (which shared hosting providers like Dreamhost don't like and kill automatically). Seems to require experienced webapp admins because knowledge of the application architecture is required.
Requires that the server support XEP-0060 (pubsub) because message transfer is built on top of it.