The official reason: scheduling conflicts. Sandberg is scheduled to speak at the Cannes Lions advertising industry conference this week in Cannes, France. Facebook did not explain Zuckerberg's conflict.

Among the CEOs of the five most-valuable U.S. companies by market worth, Facebook CEO Mark Zuckerberg was alone in spurning the invitation. Chief Operating Officer Sheryl Sandberg, who attended a December meeting with President Trump, also did not attend.

was the one big tech company that declined to send executives to a meeting about technology with the Trump White House on Monday.

Facebook board member Peter Thiel was scheduled to attend the meeting of the newly-formed American Technology Council, led by Trump's son-in-law Jared Kushner.

In a statement preceding the meeting, Kushner said the U.S. must move beyond "floppy disks" and "Y2K."

Facebook execs have been critical of Trump on some fronts. In January, Sandberg criticized the administration's travel ban on citizens from several Mideast nations, saying that they "defy the heart and values that define the best of our nation." And Zuckerberg said in a June 1 post on Facebook that Trump's decision to pull the U.S. out of the Paris climate accord "is bad for the environment, bad for the economy, and it puts our children's future at risk."

However, several of the other execs in attendance had similar criticisms, including Apple CEO Tim Cook, who said in an email to Apple employees that Trump's withdrawal from the Paris accord was "wrong for our planet," and Amazon CEO Jeff Bezos, who came out against the travel ban.

Here's a full list of all the tech execs scheduled to attend.