Google has just updated its DoubleClick ad planner, which posts new site statistics for the world’s 1000 largest websites (according to Google.) The table indicates that Facebook’s user growth is stagnating, but existing users are spending even more time on the site and are viewing almost 1500 pages per month on average.

Google now estimates that about 590 million unique users on Facebook are requesting a staggering 840 billion pages every month, excluding application pages, which translates to 1423 pages per user per month. 38.5% of the users on the Internet are using Facebook in some way. The average user visits Facebook 41 times per month, which gives the site 24 billion visits per month.

Unique users are flat with the count in December, which was down from 600 million in November. That number seems to have peaked for now, but Google believes that more than 300 million people are visiting Facebook now every day. That number is up from about 170 million one year ago. It appears that Facebook’s users are also spending more time on the site, which is now at 25:00 minutes, up from 23:20 minutes in December. The combined time on Facebook is 28,063 user years per month.

Facebook’s own statistics currently state that the company has more than 500 million active users who spend more than 700 billion minutes per month on the site.

YouTube is catching up with Facebook, at least  in unique users. Google now estimates YouTube’s reach at 500 million users and 32.2% of the Internet. Those 500 million users viewed 92 billion pages in January – which is about 184 pages per user per month. The average user returns to YouTube 14 times per month and spends about 14 minutes on the site.

Both sites are far ahead of Twitter, whose audience is currently estimated at 89 million unique users, who generate 5.8 billion pageviews (65 pages per user). Twitter users return to the site 11 times per month and spend 11:50 minutes using the service.