Facebook offers a 45% cost-per-click discount for ads that link to another Facebook page rather than an external site, according to research by TBG.

This has led to brands building a presence within Facebook so they can benefit from these lower costs.

The new research, released today, is based on 326bn impressions for 266 clients across 205 countries.

The study also found that CPCs in the US increased by 55.7% between Thanksgiving and December 17. However, CPC price for Facebook ads dropped by 11% in the UK between the third and fourth quarters of 2011.

The drop follows two quarters of rising CPC prices in the UK, and contrasts a 1% rise across the UK, US, German, French and Canadian markets.

Average cost per thousand rates were flat in the UK in the same period, compared to an increase of 8% across the five markets.

The report also found that CTR increased by 18% from Q1 to Q4 in 2011, which TBG says suggests that advertisers are building better ad creatives and taking advantage of Facebook’s targeting options.

CTR differs greatly between sectors though, as the graph below demonstrates.

Facebook has been seeking to drive up its advertising revenue before an IPO, which is rumoured to be scheduled for early this year.

Last week it started placing ads in the news feed, so users see ads based on what brands they and their friends have ‘liked’.