Do any hypercasual gaming companies buy exclusively from Facebook and forego install attribution?

That's a really good question.

If a company ONLY uses Facebook, the additioinal costs for an attribution solution doesn't really make sense in my opinion. There is no deduplication amongst various vendors to be made, and most of the services and features offered by an MMP would likely not be useful for a company at a phase where only facebook is enough / meeting your scale needs.

Up to a certain scale, a company should be able to reach their target effective cost per install via Facebook. At some point, upon scaling, you may notice that you are unable to meet your scaling needs at the same CPIs.

That is usually a good time to setup an attribution solution and bring on Google as a 2nd provider.

The moment you have more than 1 provider, it is highly recommended that you will have a neutral attribution provider, if only to differentiate the overlap the platforms will have.

The attribution companies will sometimes have a 5-50% discrepancy between their count and the count of the self reporting platforms.

This not because they have a bad platform - the contrary. This is because the amount of overlappinig attribution the large platforms have over one another due to their reach is very high.

There's definitely a lot more spend on networks outside of Facebook than in other genres primarily because the targeted audience is much broader and FB is relatively expensive.

The other primary issue is that you still need a way to associate the ad revenue that comes from a hypercasual game to the channel.

It doesn't make a big sense to work only with Facebook as your main goal is to drive volume. Applovin and Ironsource could provide you with more installs than Facebook even in US.