The number 1 is Facebook, YouTube comes in at number 3, demonstrating the immense opportunity of linking Google’s services through the G+ social layer.
Twitter is number 4, and is also the fastest growing network.
For bloggers and webmasters, it's key to integrated with the Google product set in the future.
Here's the chart from Global Web Index:
|(chart) List of Top Social Network Active Usage, by Global Web Index|