The Electoral Commission used the social network site last year to encourage teenagers to get involved, with 15,000 going on to claim their right to vote.
But analysis of electoral roll data from hundreds of council areas by global information services company Experian found the total number is falling.
Despite the overall numbers of adults registered to vote rising by 0.8 per cent to 47.5 million, only 520,000 of those turned 18 in 2011, just over 3 per cent fewer than in that group in 2010.
That compares with an estimated 1.05 million 18-year-olds with Facebook accounts.
A commission spokeswoman said: “The low voter-registration rate is very concerning. Our own research showed that only 56 per cent of 19 to 24-year-olds and 55 per cent of 17 and 18-year-olds are on the register.
“Each year the Electoral Commission runs public awareness campaigns to encourage people to register. Young people are an important group to target which is why social media has become an increasingly important platform for these campaigns.”
Jonathan Westley, managing director of Experian’s consumer information services business in the UK, said the data “underlines why it’s important that organisations adopt new technologies to better serve this section of the population”.
He added: “It is also vital to educate this demographic on the importance of registering to vote and explore how social media engagement could be part of this.”