Buddy List (buddylist.module) gives an instant-messenger-like buddy list, where you can identify users in that Drupal server as "buddies" - friends or acquaintances. It then provides you with a list of your buddies, and a page with recent blog posts from all of your buddies. It also provides an RSS feed of your buddies' blog posts, so you can just subscribe to the users you care to listen to, rather than subscribing to the whole site, or subscribing to a bunch of individual RSS feeds (one per buddy). Very cool. This was the biggest draw for Elgg in my mind, and it's now mostly available in Drupal. You can't currently define the type relationship with a buddy (co-worker, classmate, friend, instructor, etc...) but it's a start.
The FOAF module (foaf.module) is more of a sounds-cool-so-let's-install-and-see-what-we-can-do kind of thing. It generates a FOAF XML rdf file describing your buddies. Not sure how that will be used yet, but it at least opens the door to some cool relationship-leveraging stuff.
Drupal is really starting to fire on all cylinders for the weblogs.ucalgary.ca service. There are still a few things I want to tweak, but it's pretty darned close to an ideal social blogging service. Initially, I saw the everyone-blogs-in-one-place style of Drupal as a disadvantage, but when combined with the blog_theme.module that lets you customize the theme for each blog, I think the potential benefits outweigh the downside by an order of magnitude or three.
Update: I cross-posted this to DrupalEd.org on Boris' request.