Since few years ago, the Centre for Learning & Performance Technologies @C4LPT conducts a survey on tools and apps for learning and ranks them according to the votes. On 13 November 2011 it published 5th annual survey of learning tools.
As for the past two years, Twitter is number one, then YouTube, Google Docs, Blogging tools, sharing tools and social bookmarking tools. This is quite what I was expecting according to my experience. Especially, Twitter has got its popularity as a connecting and networking tool used very widely in scientific events like conferences and also in online communities to keep following the activities more easily.