That's nearly a third of all UK mobile internet users, according to GSMA Mobile Media Metrics.
Just under five million UK mobile users visited Facebook's mobile site in December, according to new research from the GSMA and comScore.
They've teamed up for a new Mobile Media Metrics (MMM) service, in partnership with the five UK operators. It will measure mobile media consumption across all their customers.
Today saw the first release of data from the survey, focusing on UK mobile internet usage. 15.9 million people in the UK used the mobile internet in December, meaning that more than 31% of them visited Facebook.
Google's sites were the second most popular destination, with 4.6 million users, followed by the portals for O2/Telefonica (3.7m), Orange (3.6m) and Voafone (3.3m).
The Top 10 is rounded out by Yahoo sites (2m), BBC sites (1.9m), Microsoft sites (1.6m), Apple (1.5m) and Nokia (1.1m).
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These top ten sites accounted for 70% of total pages viewed and total time spent online from mobiles during the month.
comScore says that while smartphone users account for 29% of UK mobile internet users, they were responsible for 47% of page views and 51% of total time spent online in December.
The GSMA and comScore are planning a host of MMM products to measure mobile usage of apps, search, reach and frequency, ad tracking and ad effectiveness.
"On behalf of our operator partners, comScore and the GSMA, we are excited to be launching Mobile Media Metrics in the UK, our first market, and we anticipate that it will accelerate growth in the mobile advertising market," says the GSMA's Rob Conway.
"The underlying principle of Mobile Media Metrics is to deliver valuable and actionable reporting tools to the media industry, while respecting the privacy of individuals."
