The reason for me to book mark a site is too get visitors from these sites. As sites like facebook, twitter, digg etc are very good source of traffic and by book marking these sites yuou you can also have a good amount of traffic for your site.
This is an good idea for a thread. I only bookmark something on the web when:
a) It contains content I think that I might want to use as reference at some time in the future.
b) I'm short on time, but would like to come back and read the content at some time in the future.
c) I expect that more content of interest to me will be added in the future (such as with a blog or news site).
BTW, out of curiosity, I just checked and I have 16,032 bookmarks.
Bookmarks