Today, numerous news outlets ran new data showing Facebook has become the UK’s most popular social network. Huh? I thought Bebo claimed that honour? Despite today’s announcement, which showed Facebook overtook MySpace in August to claim the top spot, things are a little more complicated. Everyone knows Facebook’s stratospheric growth since opening to non-students last October, but understanding the true winners and losers in UK social networking stakes depends on which measurement agency you choose to believe.

SEE ALSO: Round-Up: Bebo Vs Google; Govt’s Convergence Think-Tank, Emap Women’s Portal

Here is Nielsen//NetRatings’ snapshot of August data, the set on which today’s claims are predicated…

And here is comScore’s August report on the same sector, which I obtained from them today…

The two reports observe greatly differing fortunes for the main social nets. Bebo is either first on 10.7 million unique UK monthly users or third on just 4.4 million, Facebook is either third on 9 million users or the leader with 6.5 million - Niselen//NetRatings and comScore at least agree that MySpace is in second place (it was the leader with the former until August) - but the gap between each’s recording is some 3.7 million users. And that’s before you consider reports from a third measurement agency, Hitwise. Why the difference? Nielsen//NetRatings analyst Alex Burmaster told me…

—His firm monitors internet users age two and up whilst comScore observes only over 15s.
—comScore’s data can sometimes be skewed toward heavy web users because it uses only online polling whilst Nielsen//NetRatings also does random-digit phone polling.
—comScore monitoring software distributed via the likes of screensaver and game download sites can sometimes tilt results toward teenagers (Bebo is reported to be more popular with youngsters than Facebook).

Burmaster rubbished stats that give Bebo a lead: “Anyone who knows anything about social networking knows that’s not the case - it’s Facebook or MySpace. Their data would suggest one in three people online in the UK visits Bebo, that doesn’t sound right intuitively.” comScore’s description of its methodology states that it does conduct both online and offline polling.