By Alexandra Bruell and Suzanne Vranica

Facebook Inc. has been quietly working to prepare Madison Avenue for the long-anticipated ruling by its oversight board on whether the company should reinstate former President Donald Trump's Facebook and Instagram accounts.

Over the past few weeks, Facebook has reached out to advertising agencies in calls and emails to describe the board's process and emphasize that its management team has no sway over the board's decision, which is expected Wednesday, ad executives said.

During one call, Facebook policy executives seemed to be trying to gauge how advertisers would react to different possible decisions by the board, an ad executive said.

On Monday, Facebook sent an email to agency partners and some marketers alerting them that the decision was coming Wednesday.

"We've long stated that we should not make so many important decisions about free expression and safety on our own," the email said, according to a copy reviewed by The Wall Street Journal. "That's why we created the Oversight Board, a global body of experts and civic leaders from around the world that exercises independent judgment to make binding decisions on whether specific content should be allowed on the platform."

In the email, Facebook offered to make its policy team available on Friday to agencies, if they have questions.

On Tuesday, Facebook declined to comment.

Facebook suspended Mr. Trump early this year, saying his posts violated the company's rules against praise and support for the Jan. 6 Capitol Hill riot. Critics of the move said Facebook was stifling free speech. Facebook eventually referred the issue of whether to reinstate Mr. Trump to its oversight board, a 20-person independent body that reviews appeals to its decisions.

Advertising executives don't expect most brands to suddenly abandon Facebook if Mr. Trump is reinstated.

"If a brand was to jump off of Facebook because they felt it was inappropriate for Trump to be on, they could potentially alienate the 75 million people that voted for him," said Barry Lowenthal, chief executive officer of Media Kitchen, an ad-buying firm owned by MDC Partners Inc.

Moreover, advertisers are wary of cutting Facebook ad spending because they regard it as effective in generating sales. That draw has kept dollars flowing to Facebook despite repeated controversies. Ad revenue, which accounts for the lion's share of Facebook's total revenue, rose 46% in the first quarter compared with a year earlier to $25.44 billion.

"Very few brands have the financial security" to cut off Facebook, Mr. Lowenthal added.

Last July, the Anti-Defamation League and the NAACP called for brands to boycott Facebook, arguing that the social media giant hadn't made enough progress in enforcing its policies on hate speech and misinformation.

Many marketers did pause ad spending on Facebook, but that didn't appear to significantly affect the company's financial performance, since Facebook sells ad space to so many businesses. Last year, Facebook said it had more than 10 million active advertisers in the third quarter, up from 8 million in January.

During a speech to a marketing trade group last month, Jonathan Greenblatt, chief executive officer of the Anti-Defamation League, urged advertisers to continue to pressure tech and media companies to stop spreading hate and misinformation.

"Through your advertising muscle, you have a distinct and powerful voice in what information networks like Fox and Facebook will and will not allow on their platforms," Mr. Greenblatt said during the meeting, according to a copy of his speech. "You can hold them accountable like few other actors in society because your dollars are the fuel that enables their business model."

If Mr. Trump is reinstated on Facebook, advertisers are likely to step up their push to get more control over where their ads appear on the platform, agency executives said. Facebook promised advertisers more tools and transparency after last year's boycott. The company said it has been testing new controls that let marketers keep their ads away from topics they want to avoid in the news feed.

Advertisers already have such controls for ads on other parts of Facebook, including those in videos.

Write to Alexandra Bruell at alexandra.bruell@wsj.com and Suzanne Vranica at suzanne.vranica@wsj.com

(END) Dow Jones Newswires

05-04-21 1420ET