We do have a problem because Facebook and Gmail are huge, and because people are starting to assume the convenience and size of them somehow means Facebook and Gmail should be considered public services and regulated as such.
While I see the same problem you do- that we're all at risk of losing the correspondence and history that we care about, man do I think your reaction is super weird.
Facebook and Gmail are private web sites, and always have been. You are mistaking them for public services, when they are not and never were public services. Your mistake in no way means that laws should change or that Facebook should be required to archive and provide you access in perpetuity. You made a choice, and if you realize now that your choice comes with implications you don't like, you should start correcting that choice yourself. You should collect and gather your digital assets out from Facebook and Gmail, and archive them yourself.
Facebook is not the post office. If you want the post office, use the post office. The post office services cost you money, and Facebook's don't. The post office does not archive your correspondence at all, while Facebook does. The post office delivers physical goods, while Facebook only delivers digital goods. I am just not seeing any reasonable way to conclude that you can treat Facebook like the post office in order to suggest that Facebook be required to provide you what you wish for.
Your language is so strange to me - "That shouldn't give them unilateral rights to my stuff". You gave them your stuff. If you don't want them to have it, don't give it to them.
I'm completely lost as to how the laws are unfairly balanced in this situation. I'm normally in favor of all kinds of regulation, but this just seems unreasonable to me to suggest that we should have laws that require Facebook to provide you with permanent archive access. Do you want to pay for that archive access, or are you demanding that the government ensure that Facebook has to pay for it forever? The post office isn't required to keep archives for you, how how does your analogy work here?