In the always-beta Web2.0 world, there is a tendency for sites to never be finished. But the same is true in the world of commercial software, where it is in the owners commercial interest (allegedly) to have a release schedule which provides new functionality over time, requiring upgrades and new investment in the software.
We (I) have pretty much decided to use Drupal as the platform for our Virtual Research Environment. The number of excellent plugins for it mean that making a feature rich social networking site should be relatively easy. I have some concerns about the differing rates of upgrades, and I am also aware that politically within the institution there is a tendency to prefer 'paid for' software – but the power and flexibility of the system is hard to argue with, in my opinion.