By Astrid Wendlandt

PARIS, Nov 23 (Reuters) - Targeted online advertising is set to face increased scrutiny from European Union regulators concerned about invasion of privacy, threatening the growth of a potentially big online revenue-booster for media companies.

"This is a very hot topic that can be expected to be part of our work programme next year," Gabriele Loewnau, a senior legal adviser for the German Federal Commissioner for Data Protection, said on Friday.

The German commissioner currently heads the European Union's advisory body on data protection matters, the so-called Article 29 Working Party.

Recommendations from the working party have been used by the European Commission to get Google (GOOG.O: Quote, Profile, Research) to curtail the amount of time it stored past Web searches to 18 months.

When an individual makes an online search or purchase, the computer can remember the entry through so-called cookies and pass on the information to advertisers.

"Targeted advertising will become a major tool for us, it will irrigate various parts of the group and help increase our average revenue per user," Jean-Bernard Levy, chief executive of telecoms and media group Vivendi (VIV.PA: Quote, Profile, Research), told a telecoms conference in Montpellier last week.

Brussels' heightened awareness comes as more than 13,000 Facebook users have signed a petition protesting against the networking site's new advertising system which alerts members of friends' purchases online.

Members can opt for their transactions to be kept private but critics say the option is easily missed.