This sounds like a case where they possibly had no idea that their property was being abused this way. They might have rented to one family or person and those tenants were the ones who brought in others without the owners' knowledge. It's not like they could drive past the house and check up on things. Frankly, your mistake as a neighbor was not calling the police and/or the appropriate city or county enforcement agencies that handle issues such as homes violating local laws on numbers of occupants. You would have been doing the owners a favor, and yourself a benefit, if you had done that rather than expecting the owners to deal with it from thousands of miles away.
As the saying goes: "Out of sight is out of mind." Your FB notes to them probably got put aside in their minds, but you were right there next to the property and having issues -- why didn't you look up the local laws on occupancy and make your own life better by blowing the whistle on the tenants to the county or city? There are laws to deal with not only too many occupants but also abandoned vehicles, lawns that aren't mown, etc. I agree -- I'd be livid like you were, but saying the owners "allowed some pretty messed up stuff to go on" is throwing 100 percent of this on them as if they welcomed and permitted it freely. (Were you and they really that friendly before they left, if you assume the very worst of them like this?) I understand feeling that the landlord is to blame but (a) you knew them and didn't help them by calling the cops and/or other authorities and (b) you seem to have an attitude that they intentionally let this happen.
I would just tell them, "Wow, I am glad to have you back and I hope you're back to stay. The tenants in your house were awful neighbors, to be honest, and I look forward to living near a house where there's only one family and no cars everywhere." I'd be extra nice to them, rather than hashing out something that is done and over with. I bet they are returning to a trashed house and yard, too, and that will be their punishment as well.
If they stay a short time and tell you they plan to move again -- Then I would say sit down with them and explain the extent of the issues. Say that this time you can't live with what happened last time and will be calling the county or the cops daily if things go bad again. And then nicely insist that they pay a local rental agency a fee to handle this rental for them - so you have a LOCAL contact to call if there are issues next time. I' m not talking about a realtor to just handle initial rental; there are companies that handle the rental the entire time and deal with things like ensuring the property is kept up, etc. That's what is needed if they plan to rent again.