There is more to ensuring the success of a project than collecting actuals and checking for resource overallocations. Putting faces on the people assigned to the tasks in your project is often forgotten in the maze of software functionality.
OK, tweeting, facebooking, blogging, and the rest may seem like unbillable downtime, even if the subject matter is work related. But workers and their executives are beginning to appreciate the extent to which non-work related social bantering improves morale, and thus productivity, just as much as feeling invested in a project does. Some might even see a dependency of the two on each other.
Social networking tools also help uncover hidden knowledge that has difficulty surfacing through normal conversations and e-mail. And when the hidden knowledge is surfaced, it tends to stay surfaced and not disappear when team members move on.