In Bowling Alone, Robert Putnam takes a societal view of the decline of social capital and increasing personal isolation. Putnam only mentions the Internet in passing since it is not a major part of his thesis or his evidence. In Alone Together, Sherry Turkle takes a psychologist's individualistic eye view of the same problem.
I was personally more interested in the second part of this book, which is focused on social networks, than I was in the first part about robotics and A.I., although that is the part Turkle seems more committed to.
Although Turkle is meticulous (possible over-meticulous and cherry picking) in cataloging her evidence, the flaw in this book is that it is not a balanced view as it does not adequately portray the upside of what Turkle calls the "networked life". Entirely accidentally, I read the book in a non-networked household where the tools that Turkle disparages could provide a considerable increase in welfare and well being - something not reflected in this narrative. Turkle's repeated Waldon Pond references reveal this to be an attack on technology, not on the Internet per se. In addition, with her excessive claims - unwarranted by neurobiology - she lets herself down.
While I was disappointed by Alone Together (in contrast to Bowling Alone), there is some value here, both in documenting evidence of excessive Internet dependence, and in the sensible call to allow personal and communal space for reflection - something I attempt to practice personally. Turkle reiterated this call in her recent TED talk, which I suggest you watch (and maybe skip the book):
I've been encouraging students to engage in personal reflection for years, but this is a difficult problem in our education system. We can leave space for reflection, but we cannot adequately (maybe should not) assess it. And in an assessment-driven system, that leaves a gap which is not adequately filled.