It helps out decent with link popularity decently, but we found out some other good uses for them.
One was we did a small test with 3 different onlywire.com accounts.
If you do not know what onlywire.com is, its a website that allows you to post to a group of the top social bookmark sites at 1 time, and it allows you to put your "tags" which is just which keywords you wish to have the specific link target.
We took these onlywire.com accounts, and tested out various "tags" with various links to sites we were using to test. And what we found out is that if you find some good "tags" to use, your posted link will show up on themain-page of some high traffic sites.
Here is a example, if you use the tag "wii" and have 3 accounts do the same, you will show-up on the main-page of wii.yahoo.com .
So test and you will be surprised what other big sites grab the feed of these various social bookmark sites listed on onlywire.com.
As far as conversions on products, or clicks , they were low. These sites are good for some nice traffic, and if you have a blog type of site then you will probably gain some new readers , and people who comment.