Winner of Infoworld's best open source social networking platform of 2008. Not a bad endorsement.
One of the best things about Elgg is its easy plugin-in based widgets and additions. Want a new feature? Just check out the plugin database. If what you need isn't there? Well, you can always develop your own and put it up in the plugin database yourself. At any rate, it has plenty of odds and ends available for use, so chances are that you won't have to do any coding if you don't want to.
Elgg puts more emphasis on the individual at the center of the social network, as opposed to a more amorphous, cloudlike view of social interactions. The individual user can customize their own page and profile to a considerable degree, though the developer still has ultimate control.
Elgg may be viewed in many formats, including on most cell phones and iPhones, and is built primarily of LAMP, including MySQL and PHP.