My answer to that question is much more simplistic, albeit perhaps more difficult to accomplish, than the other responses thus far.
Obtain institutional buy-in from all of the various agencies.
Currently, I assume most of the members of GovLoop are those willing to "color outside the lines" and do so without looking for permission from their respective agencies. In my experience, that population represents a very small percentage of an organizations employees.
The bulk of the rest are not willing to do something, such as join a site like this, without some overt policy statement informing them it is acceptable.
For GovLoop to grow and become mainstream, it needs an official Stamp of Approval from each of the agencies. This would not be hard to accomplish, given the current push for transparency in government and support for collaboration from the Obama administration.
An Executive Order, issued to each of the agencies, directing them to open this site up to their employees and encourage its use would probably be sufficient.
For example, I know there are many military installations within DoD where this site is not accessible because it is Ning based.