This was due to the fact Greenhouse was depending on Spring Social BUILD-SNAPSHOT, and recent Social API changes broke the app. I've since fixed this by having Greenhouse simply use Social 1.0.0.M2 until we complete the M3 upgrade which is in progress. There is now another issue where some Greenhouse functionality fails to work due to a bug in the latest Spring Framework 3.1.0.BUILD-SNAPSHOT... this bug has been reported and I anticipate it'll be fixed shortly.
Greenhouse is pushing the envelope with both SPR 3.1 and Social/Mobile/Android, so it's likely there will be some bumps like this through the milestone phase of these projects. We'll do our best to keep master working completely.
Core Spring Development Team