Linden started the themed community program over a year ago as a way to help residents with shared interests come together to build communities. Many of the officially supported communities survived and flourish to this day: cyberpunkish "Nexus Prime" (aka Tyrell or NexCorp - located in Gibson and Bonifacio), classic "Venice" (in Bonifacio), medieval "Dark Wood" (in Dore and Dark Wood), and the airship community "Kazenojin" (in Gray). If you haven't seen them recently, you should take a look.
Tools developed in the mean time have allowed the communities to thrive with much less help. The primary help that Linden gave the communities was to reserve part of a sim to prevent others from claiming land while the community grew. This was critical in the days when someone owning a small plot could use up all the objects allowed in a sim. The program started before the existence of group-owned land and each of the current themed communities have made use of this feature.
We hope each of these communities continues to grow. Linden plans to add features that make will group management much better and we will look for other systems to make it easier for you to join with others and build special communities together. Your suggestions about how to do this in a scalable and equitable way are welcome.