It's open source (laconi.ca), scalable, easy to install, and easy to theme/develop with. It uses the openmicroblogging protocol giving you the ability to subscribe to users on other services and receive their updates, etc. It allows image uploading via other services APIs from the site itself. It has a twitter compatible API, you can subscribe to your twitter friends, as well as cross post to twitter, etc. It has XMPP, Email, SMS, API gateways. URL shortening, the ability to create groups, etc, etc. I've been following it's development off and on for about a year now, and great project. I don't use twitter or laconica/identica myself, so the usefulness of it is arguable. Regardless, I've had fun playing with the code.