Facebook and social networking developer have entered the top 10 this year, at the expense of Friends Reunited and Google Video, which dropped to 16 and 14 respectively.
YouTube took the number one spot with 10,426,000 UK unique audience in January 2008, 56% more than last year; while Wikipedia dropped to second with 9,557,000.
Facebook has taken the number three spot, followed by Google's Blogger, MySpace, Bebo, Slide, Yahoo! Answers, Windows Live Spaces and TripAdvisor.
Meanwhile, social network PerfSpot is the fastest growing social media site over the last year, growing by 713% in just nine months, slightly more than Facebook grew across the whole year.
Half of the 10 fastest growing are video sites, led by VidShadow, which has grown by 639% in just two months. November 2007 was its first measurable month of data.
In January this year, 20.8m Britons, 63% of Britons online, visited at least one of the 10 most popular social media sites -- which is 21% more than visited at least one of January 2007's top 10.
Alex Burmaster, internet analyst at Nielsen Online, said: "While the majority of the most popular social media sites are the networks, most of the fastest growing are video sites, which points to video being the biggest star of the 2008 social media scene."
Earlier this week, Wikipedia was in the news for a different kind of social media interaction. Its co-founder Jimmy Wales publicly dumped his girlfriend, right-wing political pundit Rachel Marsden, on the site.
Wales posted a statement on Wikipedia that said: "I am no longer involved with Rachel Marsden."
In response, Marsden put a shirt and jumper of her former boyfriend up for sale on eBay with the text: "Hi, my name is Rachel and my (now ex) boyfriend broke up with me via an announcement on Wikipedia."