Most of my experience with social DRM has been through smaller tech publishers like The Pragmatic Bookshelf, where they embed the purchaser's name into the title of the book, like
<dc:title>Programming Ruby 1.9 & 2.0 (for [name])</dc:title>.
Probably the biggest titles to use social DRM, though, are the Harry Potter books through Pottermore. Here's a link to an article that goes into detail about it: http://copyrightandtechnology.com/2012/04/08/the-harry-potter-watermarking-experiment/ and a relevant chunk:
The EPUB version that I downloaded is not DRM-protected; instead it
contains two things: “This book is watermarked and was acquired by
user ec107c00b9577436d6354e54cd9da5c9 on 31 March 2012″ on the
copyright page, and various bits of data inserted invisibly into
images and other places inside the book.
As far as pros and cons of social DRM vs. more traditional DRM: there's lots of info out there that suggests that DRM in general is not terribly useful, but there are strong feelings on both sides of the issue. I'd suggest checking out what tech publisher Tim O'Reilly has to say: http://www.forbes.com/forbes/2011/0411/focus-tim-oreilly-media-e-book-antipiracy-steal-this.html.