The writing on the wall is clear: social media will soon be the dominant internet activity.

In another milestone Facebook has pulled ahead of Yahoo as the biggest site in the U.S., according to Compete.com, at about the 133 million unique user mark. And as Facebook continues to rise, Yahoo has not simply plateaued, but fallen from its height of 140 million in June 2009, a sign that all the traffic growth is happening in social.

Google is still at the top, meanwhile, with more than 147 million unique visitors, according to Compete.com. But Facebook has already passed them in visits, 2.8 billion to Google’s 2.78 billion, and, more importantly for advertisers, Facebook fans remain on the site three times longer than they do on Google properties, Compete.com says. No wonder Google rushed out Buzz last week.

Meanwhile, in the world of microblogging, Twitter,  after a brief flatline, continues its meteoric rise. In January, it attracted 73.5 million unique individuals, according to comScore – an 8% increase on December. But perhaps the most astounding figure is that total visits to Twitter has grown by a factor of more than ten in a single year.

Pay attention, brands. This is where more and more of your customers are spending time. And they keep spending more and more time there.