Facebook's advert library shows that the Conservatives have been testing a variety of subtly different messages.
The practice is not unusual, but has become much easier to spot using Facebook's transparency tools.
Other political parties, including the Labour Party, have also run similar tests in the past.
"It is extremely common now, particularly among people who sell things online," said Rob Blackie, a digital strategist who works with non-profit organisations and political parties.
"It's a good idea to do this. Typically, in every test you do, you find something that makes it 1% or 2% more efficient.
"Accumulatively over time that can add up to an ad that is 40% or 50% more efficient."
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The adverts for Mr Johnson were highlighted by Sky News correspondent Rowland Manthorpe, who suggested the party may be "gathering data for an election".
Posting several slightly different adverts can help political parties learn which messages are resonating with voters, before spending more money promoting the most successful adverts.
However, on Thursday, Mr Johnson denied he would call a general election, saying there had been "enough elections".
The Conservatives are far from the first political party to test their advertising messages in this way.
The Cambridge Analytica data scandal revealed how Vote Leave had micro-targeted political adverts at Facebook users during the 2016 referendum.
"Until very recently, you couldn't see what people were doing," said Mr Blackie.
"But Facebook now allows you to see what ads people are putting up.
"It has become more prominent with political parties, most campaigns have started to do this - although to do it well at scale can be quite time-consuming.
"Specialised software can automatically create different combinations and pick out the winners, and shut down things that aren't working."
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