Between April 2007 and April 2008 the number of minutes Nielsen estimates the UK population to spend on Facebook has surged from 500m to 2.4bn.
The number of minutes spent on MSN/Windows Live Messenger fell over the same period from 3.2bn to 2.4bn.
Nielsen figures also show that in general instant messaging is losing ground to people communicating through social networking. Instant messaging minutes were down from 3.9bn to 2.9bn while social networking minutes were up from 2.4bn to 3.7bn.
Alex Burmaster, internet analyst at Nielsen Online, said: "Instant messaging has already seen its mantle as the UK's most engaging online sector pass to member communities and it seems that the dominance of MSN Messenger as the most engaging brand is shortly to be ended by Facebook.
"This has been a seismic shift in a relatively short space of time and illustrates how social networks have fundamentally altered the way people communicate online.
"It will be interesting to see what further effects Facebook's IM application will have on the current players in the space."
Burmaster pointed out that consumers are not spending significantly more time online than they were a year ago -- total minutes are 33.3bn, up 4% from April 2007, which means that if one sector grows another tends to shrink.
After MSN Messenger and Facebook, eBay accounted for the third highest minutage, at 1.7bn, unchanged from a year ago.
Fourth was Google search, up from 700m to 900m, and YouTube, up from 400m to 600m.