This site discusses the benefits of Drupal. Drupal's extensibility and more active developer community beats Elgg in some instances. However, drupal is a more generic CMS (content management system), and so is harder to customise to social networking needs yourself. However, DrupalEd is a freely available customisation of Drupal that "does blog, podcast, wiki, social bookmark, assignment, social networking, e-portfolio, and calendar right out of the box". And being open source and modular, you can add or remove modules you don't want (assignment in our case).

Worth considering. There may even be other social networking customisations out there.