Signal explained how it went down in a blog post Tuesday. The idea was to post ads on Instagram which use the data an online advertiser may have collected about users, and basically show the user what that data might be for them.
However, Facebook wasn't having it, and shut down both the campaign and Signal's ad account. It's unclear which rule or guideline, exactly, Signal has breached in order to get banned.
See Signal tried to run a very honest ad campaign on Instagram. Facebook said no.
Instead, Signal's ad account on the platform got banned.