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.
From those who has messaging option for example (telegram chat/Facebook chat etc) can be manipulated from alt account/accounts and also they might be able to delete any negative comment/views from telegram group and they can do the same on other messaging chat. I think bitcointalk forum is the best one from where we can get enough information about a gambling site by asking on the forum if that is not listed on the forum and if listed then there is already enough information.
bitcointalk is not even only for gambling if you want to know, yes any related to gambling and you are confused of it bitcointalk you will know them very well with out moving for further things to do, i supprt you and your suggestion and again bitcointalk have diver's thing it brainstorm some body both bitcoin and country politics not only gambling