Niche down!

Find a way to appeal to a certain demographic. You can't compete with Messenger or Whatsapp when it comes to their reach.

But you can compete on features or how you position your product.

Add features to your product that the bigger products don't have. it could be something like better tools for managing a group chat, or tighter integration with a software product that your target demographic would use.

The choice of the platform, as well the launch strategy would then hinge on the user demographic you are targeting.

In terms of who to target first, it'll likely have to be the free users, just to build a base. As you add on more features, these users will likely be open to paying for them. For platforms to launch/market the product, it's important to look at the buyer persona. Where are they/what platforms do they use? You'll want to be on the same ones.

The value of a messenger app is in its adoption by heavy users who grow to depend on it, and then invite more people. You need to find a niche, and then aggressively get some adopters. Then, find out what value-added features would make them happy. By aggressively recruiting, I mean that just making a free version is not good enough. There is so much noise and competition. You need to contact users in this niche and, if need be, "bribe" them to use it. I know one app in the truck driver space that went to truck stops and gave out free t-shirts and food to get truckers to download and try their apps.