Facebook use among over-50s in the UK has risen faster than any other group, according to research firm Nielsen.
Overall, membership of the social network grew 41% between 2009 and 2011. Among older users the figure was 84%.
The trend was similar for Twitter, although unlike Facebook, younger users appear to have drifted away.
Nielsen's findings contradict research published earlier this month that suggested the site's popularity was dwindling in the UK and USA.
The latest study was commissioned by the UK Online Measurement Company (UKOM), which is funded by advertisers and media agencies.
Its measurements are based on the behaviour of panel of internet users, both at home and in work.
The research found that, in May, 26.8 million people in the UK visited Facebook, propelling it past Microsoft's combined sites - MSN, Windows Live and Bing. Only Google was more popular, said Nielsen.
Twitter enjoyed a record month, reaching 6.14 million unique visitors - a 34% increase on the previous month. The number of women aged over 65 using the site grew by 96% over the same period.
Business network LinkedIn clocked-up 3.59 million unique UK visitors in May - 57% more than in April.
If accurate, the growth among older users is likely to be welcomed by advertisers, keen to tap the wealthy demographic.
"The younger audience, under 25, monetize very badly," said Simon Spaull, chief development officer at advertising agency TBG Digital.
"They are there to waste time. Clients are more interested in return on investment and an audience they can monetize."
The idea of more older people on Facebook may spark fear among younger users, wary of mum and dad popping-up online.
But according to trends expert Dr Antonia Ward from The Future Laboratory, the broadening appeal is inevitable.
"It is a platform. As it becomes embedded in society it becomes more embedded across the general public."
"Something like Facebook has to have a very broad base of users or it does not work," she said.
That combination of established and edgy is seen as a virtue by many advertisers, said Sam Gadsby, head of social media at Clicky Media.
"LinkedIn is perfect for a business-to-business client, Twitter is an organic build.
"Facebook combines both and is definitely one of the more exciting ones because it could go viral," said Mr Gadsby.
Rise and fall
The popularity of social networking, and Facebook in particular has been a matter of some dispute in recent weeks.
One website, Inside Facebook, claimed that user numbers had fallen in the US, UK and Canada, despite growth elsewhere in the world.
Another piece of research carried out by Brilliant Media, based on Google Analytics data, appeared to confirm the Inside Facebook findings.
Facebook does not comment on statistics generated by third-party research.