It’s either an inexcusable invasion of privacy or a vital means of monetising content, depending on your side of the fence—but there’s no denying that behavioural targeting advertising is now being taken very seriously.

A new report from Forrester Research shows behavioural is growing fast in western Europe: just 10 percent of European companies Forrester surveyed in 2007 said they used targeting but that number grew to 26 percent in 2008, when the report says behavioural “finally took off” after years of reluctance from advertisers - 58 percent of advertisers surveyed say they are now interested in using behavioural ads in 2009.

The practice of tracking user behaviour is a touchy one for many web users who feel that behavioural trackers invade privacy. But the issue is bigger than controversial Phorm alone, as the report notes: “Advanced targeting has become standard in mainstream ad servers such as Google’s DoubleClick DART and Microsoft’s Atlas”.

—Popular behaviour: Last year, 42 percent of companies opted for contextual advertising—although with Google (NSDQ: GOOG) now automatically including new clients into its contextual programme, the real number is much higher. The report says 39 percent of companies used demographic targeting last year—though researchers found that many class this as manually choosing the kind of site ads will work best on; 37 percent of companies reported using geo-targeting last year.

—Unexpected results: Dave Morgan, former CEO of Tacoda, told researchers that behavioural generates some unexpected datapoints: he says that the users most likely to respond to ads for flat-screen TVs are not people who have researched TVs online but those who like reading military news and that people who watch porn online were among the most likely to respond to banners for car rental services.

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