Other than the messaging platforms that you said, we can also post the problem directly to the public chat of a gambling sites so that other players are going to get aware of the issue. Some gambling sites also have their own communities or forums, you can also create a topic there.
I think the chats that are open on specific gambling sites can be moderated quite heavily. Therefore, the overall picture may not be entirely objective. Because the moderators, quite obviously, will erase posts with negative reviews, or leave the most harmless ones, so that there is an appearance of objectivity.