Pligg is probably the most popular and sufficient social bookmaking open source content management system.
There are many popular websites that are based on Pligg and some of these actually look like full fledged social networks.
Web Server Requirements
Basically Pligg need Apache or Nginx based web-server with PHP 5 and MySQL 5 to run ( Pligg version 2). As nature of social bookmarking websites, a good minimum will be mid-class VPS to be able to handle traffic and high server load.
Installation is very easy and experienced webmaster will need no more than 5 minutes to complete it.
Pligg is very flexible CMS that can easily be customized to fit your needs, additionally there are many free and paid modules, widgets and themes. Latest version design is based on Bootstrap 3 and other out of the box features are:
Pros and Cons
I can't say that there are any cons, mostly because there isn't other open source social bookmarking CMS that it is regularly updated and it is based on latest. Pligg is sole leader on this field. Probably the only disadvantage is that Pligg websites can get easily spammed, but as soon as security is tight up, this problem will drop automatically. Pligg is powerful CMS and number of free and paid modules can turn it into almost any kind of web project.
Previous version used to have some troubles with speed and high server load. At WebmasiterPro, we've tested Pligg v2 on shared hosting and it seems that everything was running smoothly during our stress tests.
If you would like to build social network alike website or simply social bookmark website, Pligg is probably the best choice for CMS based on PHP.