The demise of Google's Wave is a stark reminder of just how hard it can be to launch a new technology idea:

http://www.theregister.co.uk/2010/08/04/google_wave_dead/

A key take home lesson for me is to try to make the success of SP less dependent on the network effect. Hopefully SP can be more like the mobile phone rather than the original telephone (mobility is useful for one user even if everyone you talk to has a land line).