As millions of Canadians cast ballots in pivotal provincial elections this year, it will be difficult, if not impossible, to gauge how much they were influenced by online campaign ads tailored to their specific interests and opinions.

Political parties and candidates are increasingly relying on Facebook in particular to communicate their sales pitches to voters, and election observers are concerned about the lack of transparency about who's behind ads.

With its massive trove of data, the social media platform allows campaigns to microtarget to different groups of voters based on factors such as age, gender, geographic location and political interests and attitudes.

It's why you might see an ad from a candidate about health care spending while your friend sees one from the same candidate about cutting taxes.

Christine Elliott, a candidate in the recent Ontario PC party leadership election, placed this ad on Facebook. She lost to Doug Ford. (Christine2018.ca)

The tactic has been used by campaigns in Canada for several years, but what's changed is the level of sophistication in customizing ads to specific voters and the sheer volume of Facebook ads now being pumped out.

"It is accelerating almost exponentially," said Dennis Matthews, vice-president at political consultancy Enterprise Canada and a conservative strategist who ran advertising for the Conservative Party of Canada in the 2015 federal election.

"I think by the time we get to the next federal election campaign the parties will be spending more on digital ads than on TV ads likely for the first time."​

Even before the 2019 federal election, voters this year will pick premiers in Ontario, Quebec and New Brunswick, and there are municipal elections scheduled across Canada.

Political advertising on the social media platform has come under increased scrutiny since the 2016 U.S. election, when it is widely believed the platform was used by Russian operatives to influence voter attitudes. Facebook has faced pressure to make it easier for users to understand who is targeting them with political messages and why.

Cost-effective for campaigns

Advertising on social media platforms will be key to reaching voters, particularly younger ones who tend to not read newspapers or have cable TV, and prefer podcasts to live radio.

Despite Facebook coming under fire for the Cambridge Analytica data breach, political strategists expect political advertising on the platform to keep growing.

"In terms of variety of messages, and issue-specific messages, I see the current and future platform for that as being primarily digital," said Kate Harrison, a senior consultant at political consulting firm Summa Strategies in Ottawa who has worked on digital strategy for Ontario provincial campaigns.

Facebook CEO Mark Zuckerberg was on the hot seat, testifying last week on Capitol Hill about how it uses user data. Facebook has also been criticized for how it handled political advertising in the 2016 U.S. election. (Leah Millis/Reuters)

​Harrison said Facebook advertising is extremely cost-effective, and campaigns love how specific they can be in tailoring their messages, and how they can test them out and tweak them if necessary.

"You can see what's being clicked on quickly or not at all," said Harrison. "They're cheap to develop, pretty cheap to post, and you get-real time data and insights on what messages are performing better than others."

All that is great for campaigns, but some experts warn this kind of advertising can weaken the democratic process.

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Facebook and U.S. intelligence agencies said that during the 2016 U.S. presidential campaign some ads were purchased by Russian operatives seeking to influence the election, often by featuring divisive issues.

Those who research and report on political campaigns are trying to adapt to the phenomenon of microtargeted ads, which make it more difficult to fact-check politicians' statements and track the ways they are appealing to voters.

"It causes issues when you have to cover a campaign if you want to do an exhaustive evaluation and examination of what type of political communication parties are putting out there so you can report it more effectively to citizens," Thierry Giasson, a political science professor at Laval University, told CBC News.

"It also causes serious problems for researchers and people like me who are engaged in analyzing content in a systematic way, and across time," said Giasson, who specializes in political marketing and communication.

CBC News to track ads with crowdsourcing tool

Media outlets in the U.S. and now in Canada are trying to find ways to monitor Facebook ads.

CBC News is partnering with ProPublica, a U.S.-based independent investigative journalism organization that built a crowdsourcing tool to track paid political advertisements on Facebook by having users submit them to a database using a Chrome or Firefox browser extension.

Watch the crowdsourcing tool in action and find out how you can participate:

Learn more about CBC's effort to track political ads on Facebook through crowdsourcing. 0:50

Facebook said it is taking its own steps to increase transparency in political ads, and last year announced new requirements, being tested in Canada first, that ads be identified as political in nature and disclose who bought them.

The rules will apply to paid advertising only; they won't capture other political content that could potentially influence voters on the social media platform. Politically minded people can create their own pages, for example, and create graphics and write posts that can be shared among users. That kind of content isn't subject to advertising rules.

Giasson, the Laval professor, said all political parties now design their campaign strategies with microtargeted ads on Facebook in mind.

And he said there is another downside to this kind of campaigning that is becoming so prevalent in Canada, and around the world.

"It diminishes the common good and it diminishes the idea that we are all sharing a common political experience, which is an election," said Giasson.​