I think it's possible but unlikely first and foremost bc FB has financial incentive to create a fair advertising ecosystem. Since FB sells inventory by auction

I think it's possible but unlikely first and foremost bc FB has financial incentive to create a fair advertising ecosystem. Since FB sells inventory by auction, they make more money when there's more competition. There's a very long tail of small advertisers on Facebook, and handicapping them would ultimately lead to lower revenue for FB. On top of that, if such favoritism were real and ever were revealed, FB would suffer huge reputational damage which just isn't worth the risk when they're sitting on a cash cow. I started out with low budgets too, and I can see how it would be easy to blame the platform when the real problem is poor campaign design and creative. Don't get me wrong, big advertisers have a huge advantage bc they can iterate on creative and campaign design faster. And, this advantage often explains why most small advertisers fail. But, at the actual auction level, I've never seen any evidence of favoritism nor do I think it would make sense from Facebook's perspective.