Questions over the social network’s role in politics are particularly raw in Britain, where outside groups were accused of spending lavishly on Facebook during a heated campaign before a referendum on the country’s membership the European Union. In response, Britain’s privacy watchdog has started an investigation into whether such targeted political advertising breached its strict data protection rules.

“Political advertising is fundamentally different; there’s a lot of concern about what’s being seen on Facebook,” said Sam Jeffers, the group’s co-founder and a former digital media strategist. “The people deserve some sense of what’s going on.”

As the volunteer group is not completely representative of the British population, the data is by no means perfect, highlighting the difficulty of tracking political activity on Facebook.

In the buildup to the election, for instance, the data showed that the Liberal Democrats — who are likely to remain a minority presence in Parliament — posted the largest number of political ads on Facebook. The Conservative Party was second, despite the political party’s pledge to spend 1 million pounds, or $1.3 million, on social media messaging. The Labour Party, which planned to spend a similar amount, was in third place.

Initially, all the British parties spent money on broad-brush messages that blanketed the social network without targeting specific voters. But as Election Day approached, that strategy began to change.

An analysis of the data by The Bureau of Investigative Journalism, a nonprofit media organization, showed the country’s major parties were increasingly targeting specific voting districts and wavering voter groups with direct Facebook ads. The number of ads seen by WhoTargetsMe? volunteers has also roughly doubled in the last month, though political messages still represented 2 percent of overall ads displayed in Facebook feeds, according to the group’s analysis.

The ads have included Conservative Party messages about potential nuclear energy jobs in three areas in northern England with ties to the industry, and that are some of the country’s most contested districts. By contrast, the Labour Party targeted older women nationwide with directed ads about potential threats to their pensions.