They're running Wordpress, which is famous for scaling badly to the extent that there are large companies like WPEngine.com devoted to doing nothing except working around that.
A lot of people like the back-end admin panel of WordPress and its WYSIWYG feature, so a bunch of other people have created a lot of themes/mods/plugins around WP because of that.
If you run a small website it's all fine, otherwise you have to deal with performance issues and WP limitations because you've decided to build a social network on top of a blogging engine.