They may be among the most-visited sites on the internet, but even this type of exposure doesn’t seem to be enough to keep the naysayers at bay. The Guardian is reporting that market research company eMarketing is to lower its social-networking sector’s ad-spend forecast by $250 million (£127 million).

The downturn in ad-spending brings the total revenue predicted by 2011 down to £2.15bn.

Fake ‘viral’ marketing to blame

Speaking about the announcement, Debra Aho Williamson, eMarketer's senior analyst, said: "Social-networking websites are still trying to figure out what sort of advertising works.

"Tapping into consumers' conversations and spreading brand awareness virally has proven more challenging than companies originally thought."