Giphy's vast library of looping short video animations is hugely popular in Facebook's apps.
But it also provides animations to competitors like TikTok, Snapchat and Twitter.
Now, the UK Competition and Markets Authority (CMA) is investigating whether the purchase is a problem.
It has sent an enforcement order to Facebook, effectively putting a hold on any merging of the companies until its investigation is over.
Announcing the acquisition in May this year, Facebook said that half of Giphy's traffic comes from Facebook apps, including Whatsapp and Instagram.