The net research firm revealed that in May 2010, just 3.3 million web users visited the social network, compared to 6.5 million in May 2009. That's a 49 percent drop.
MySpace was founded in 2003 and at its peak had 100 million members. In 2005, News Corp, which is headed by Rupert Murdoch, acquired MySpace for £351m.
However, the social network struggled to keep pace with rival Facebook, which has soared in popularity since 2008 and subsequently overtaken MySpace.
This week, Facebook announced it had reached the 500 million members mark. In the UK, the social network has 30.4 million users, which, according to ComScore, is 79 percent of the country's web-using population.
Meanwhile, micro-blogging site Twitter has 4.3 million users in the UK, which is 62 percent up on last year.
ComScore also revealed that 90 percent of UK web users aged 15 and over used social-media sites in May 2010.