There are many people that have concerns about using social networks, often due to privacy concerns, however there are good reasons for using social networks/bookmarking sites.
Reasons for using social networks/bookmarking sites:
***Reach target audience
***Notify large groups of people quickly
***Integration of various social network platforms
***Backlinks
***Possible resubmissions by users (ie ReTweeting of your Tweets), which means even more people notified