Facebook adopted a warning service to help users avoid clicking on dubious internet links, with a website ratings system developed by a Finnish start-up.

The new warning service by Helsinki-based Web of Trust (WOT) helps Facebook boost security for its more than 500 million users, amid growing concerns of online scams, spam and malware.

WOT's software calculates the reputation of 31 million web pages and updates the ratings twice an hour, based on feedback from some 20 million users.

With the new feature, an improvement to its existing safety-scanning system, a click on a link to a low-reputation website prompts a warning message. The ratings are based on the input of some 20 million people and data from information security companies that blacklist harmful websites.

Facebook declined comment on the volume of security attacks and the value of the deal with WOT was not disclosed.

WOT's investors include Michael Widenius, founder of open-source database MySQL. The start-up sees itself breaking even in the summer on turnover set to reach between 2 million euros ($2.8 million) and 3 million this year, said Vesa Perala, WOT's managing director.

Our target is to reach 50 million users by the end of this year and 100 million active users is the aim in the long term, Perala said.

(Reporting by Jussi Rosendahl; Editing by David Holmes)

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