An interesting thing about many of the properties owned by Yahoo is the tiny social elements built around each of these that helps Yahoo make its users engage better within its properties. Here are some stats that supplement this fact
Yahoo Mail and IM Status Updates : 800 million per month
Number of emails on Yahoo Mail : 100 billion per month
Photos uploaded on Flickr : 4 billion (100 million per month)
Questions and Answers posted on Yahoo Answers : 30 million per month (2.4 Q&A per second!)
Number of communities on Yahoo Groups : 10 million (120 million members)
Messages sent via Yahoo Groups : 15 billion per month