New reports from The Neilsen Company show that when it comes to keeping visitors on a site…and coming back for more…social networks are the runaway winners.  In December 2009, visitors spent an average of 5.5 hours on Facebook and Twitter for the month, up 82% from the previous year.  And that number grew to over 7 hours in January 2010. Here are the top 10 brands on the web, the...