Pligg is an open source content management system that powers thousands of websites. For those who are looking to start their own social media site, Pligg is a popular option. It’s free, fairly easy to use and customize, and it’s probably the fastest way for a new social media site to launch.
most of the pligg sites are nofollow. some also dont give you direct link form the bookmarked page and rather direct to an inner page of the site which end up being nofollow. use them if it is easier to do but i woudn't count of them too much. there are few good ones out there so if yuo can single them out and utilize them, do so
Pligg is an open source CMS (Content Management System) that you can download and use for free. Pligg CMS provides social publishing software that encourages visitors to register on your website so that they can submit content and connect with other users. The software creates websites where stories are created and voted on by members, not website editors. With Pligg content management system, you can start your own social publishing community in minutes.
thekingofseo is right, Bookmarking, you get to put your keyword as title and you can link it to your website with a description - you should always see a rise in serps when done correctly.