Social networking site Facebook accounted for over seven percent of all UK internet visits in July this year and one in every six page views, according to Hitwise.

However, Facebook, which reached the half a billion users mark last month, may be seeing its page view growth slow. According to Hitwise, the social network's market share of UK page views has trebled over the last five years but growth has slowed over the past six months.

Facebook "Last month there was a slight decline in share," said Robin Goad, research director at Hitwise, in a blog.

However, Goad added this may well be down to seasonality, as the back to school period has a big impact on Facebook's figures.

Goad also said the amount of time UK users spend on Facebook has decreased from 30 minutes in January to 27.36 minutes in July.

"Cleary Facebook is not losing traffic in the UK, but do these stats do point to a stabilisation? The rapid period of the site's growth is now probably over in the UK, but does that mean Facebook has reached saturation point?"

According to Hitwise, the UK's second most popular site by page views, after Facebook, is Google. This is followed by eBay, YouTube, Windows Live Mail, Yahoo Mail, BBC News and Sport,  and Amazon.