I had assumed FB bought Giphy in part because Google bought Tenor. It's interesting that the FB acquisition has seen more antitrust scrutiny relative to the Google acquisition.
In the realm of social media, FB is much more dominant.
Companies that are more dominant, almost by definition deserve more anti-trust scrutiny.
The closer a company is to being a monopoly, in that specific market, the more that anti-monopoly laws will come in to play.
Edit: Actually I kind of walk this back. I don't think Tenor has ever stopped providing the same service in an open manner (i.e. via their API), which would be needed to make the argument that there is a competition concern somehow.