It was a long time coming, but Twitter released their "Who to Follow" tool... and it's terrific. It makes Twitter more navigable and more socially relevant: For me at least - this translates to richer content, more frequent usage and a deeper Twitter social graph. It is powerful because the "Who to Follow" recommendations are very, very good. I am not sure what the recommendations are based on - but they are highly relevant and have a high hit rate. Below each recommended user is a list of other Twitter users who are particularly relevant... they of course add valuable context. But perhaps the most compelling part is that Twitter includes a bio for each user. This is unique because:

1) Facebook doesn't have user bios, and 2) Twitter users work hard to craft < 140 character summaries of themselves... so they are compact, promotional and interesting

In contrast, Facebook's People You May Know tool lacks the same relevance and information (there is no user bio and you have to work to find friend's in common). More importantly, Facebook doesn't seem to have as powerful of a recommendation algorithm. Now users have plenty of ways to find other users of Facebook: their Friend Finder, the Facebook feed itself, the like graph, etc: