We've just written up a case study for Drupal.org that explains some of the technical challenges that had to be overcome on a recent project of ours called Property Place. This project posed particular technical challenges due to the volume of data that we were having to deal with (350GB of images and 2GB of CSV data!) – essentially what amounts to every property for sale or rent within the UK.
If you don't fancy reading the whole case study, then to summarise the project in a single paragraph; Property Place is a Facebook application built using Drupal 7 that allows users to search, share and advertise their property. It uses Drupal Commerce, Apache Solr, Migrate and a range of other modules to enable this functionality. The application aims to be the number one Facebook property application, and although the app is focused on the UK market at present, it has global ambition and capability.
Check out the full case study over at Drupal.org or read our portfolio page entry to find out more.