Audience overlap isn't really much of a concern on Facebook -- Facebook has 1.56BN DAUs as of Q1 2019 -- but it can become problematic when you start using very broad lookalike scoping (eg. 10%) on AEO and VO campaigns that are running simultaneously (see more in this answer: What is the difference between AEO and VO bid types on Facebook?).
VO and AEO campaigns naturally tend to iteratively reduce / hone targeting to the same groups of people, and running campaigns with those types of bid strategies on the same audiences will magnify the impact of the simultaneous targeting.
EDIT: re-reading this answer a few weeks after writing it, I think the point I was trying to make isn't clear.
Audience overlap doesn't have to be / shouldn't be a concern on Facebook: Facebook's penetration is large enough that, if you are smart about campaign construction, you don't need to be concerned with audiences overlapping (and thus driving up your CPMs).
But what does being smart about campaign construction mean?
- If you are running multiple AEO / VO campaigns, you are either targeting very specific segments that are independent of each other, or you are running fundamentally different creatives;
- You aren't using broad Looaklike (eg. 5%+) audiences in the same geos across multiple campaigns;
- You aren't running test campaigns against the same custom audiences as live campaigns with significant budgets;
- With multiple AEO / VO campaigns, you are using very strong signal events (eg. purchases) and not up-funnel events like registration or tutorial complete. If you have multiple event-optimized campaigns running, make sure the events are down-funnel enough to really segment users.
The point I was trying to make with the original post was that audience overlap doesn't have to be a real liability on Facebook -- it's certainly possible to avoid that problem with just a little bit of planning and strategy. Obviously if you are spinning up multiple VO campaigns with 10% LaL audiences based on payer seeds and targeting up-funnel events in the US, you'll run into problems with overlap.