Yahoo UK has officially launched its

Answers social search tool

in what the portal says its biggest ad campaign for five years.

The Q&A service, which has been in beta since April, presents user-generated answers to queries rather than algorithm-based results.

The launch, which follows the smart purchase of last December, progresses Yahoo’s plan to use social bookmarking to regain ground on Google.

It launched the service in the US at the start of this year, where it has widely been seen as a success. Yahoo says it now has around 50 million users, while research group Comscore says it is second only to Wikipedia in usage of online reference tools.

The service earns money through contextual ads and banners, but Yahoo also uses it to drive traffic through the appearance of answer pages on other search engines.

In contrast to Google Answers, which uses a team of 500 “carefully-screened” researchers to provide paid-for information, Yahoo’s service allows anyone to submit answers and doesn’t charge those who submit queries.

Stephen Taylor, head of search and search marketing at Yahoo! Europe, told The Guardian: “We do see our core internet search and social search getting closer and closer together. Essentially, what you are building is a global knowledge database.

According to Comscore, Yahoo Answers attracted 12.3 million unique visitors in June, while 947,000 people clicked on Google Answers.

But Google is expected to mount a major challenge through Google Co-op, a site designed to build specialised search tools around specific topics such as health and motoring.

According to eWeek, it also was recently awarded a patent which suggests how the search giant plans to tackle the social search space.

It includes details of how Google plans to better judge users’ intentions when they search by matching their queries with ‘themes’ of questions commonly posed on its engine. According to the patent, editors will develop these query themes by looking at search query logs and developing categories of information.