It may increase site traffic - but how many 'bounce' immediately, and how many stop to do business.

I'd love to hear evidence that social link dropping actually boosts a business, rather than simply raises the number of brief visits by social drones.

The main benefit is linkage, IMO. If you have content that's interesting enough to go seriously social (thousands or more visits in a day), then you'll probably pick up a few links from bloggers. No case study, but I've seen plenty of anecdotal reports.

Social traffic rarely converts well, even on ad clicks, much less purchases. Still, a little extra linkage is always a good thing.

It is my hunch that whoever visits yr site due to a social bookmark link will be a "soft" visitor as you say. But I write a political blog & not a commercial site. So I'm less interested in whether someone does business at my site (though I am interested in people who will actually stay & read what I write). I'm always looking to build my readership. So I figure if even a very small percentage of those click convert to regular readers then I've helped myself. And links fr. other blogs also would help me as Rogerd notes.

But again I think your warning about such visitors not being likely to stay around or be serious about yr site is warranted.

It depends on each media you are using.

SU - Has more of fun loving crowd, so they'd cherish a funny photo or something. Their bounce ratio is also quick. They come, they see and pass on to others. Does not pass value but steady and good traffic.

Digg- has more of news based stories and more serious crowd. They probably will not like a funny image but a kickass product you developed.

Like the above each media has a different crowd, you need to be clever to manage all of them alike.