Do you think one person can not do 300 social bookmarking in a day?
They using firefox and using 25-30 tabs, while bookmarking i.e at a time they bookmarking in 25-30 websites. It basically depends on your experience and speed. At the beginning same person doing 50-100 bookmarkings, but as time goes, they know the tricks and speed up process.
That benefit you can get when you have experience staffs.