Yep, this is how pretty much every large data warehouse works as well. It's very useful to be able to say "What was the state of everything at time X", even if it's been deleted you want to be able to see the state at certain times.
I also imagine with all the data that FB has, they probably have a legal obligation to keep data as it can be useful in solving crimes or gathering evidence. In fact their TOS explicitly states that they will preserve data: "We may access, preserve and share your information in response to a legal request"
I also imagine with all the data that FB has, they probably have a legal obligation to keep data as it can be useful in solving crimes or gathering evidence. In fact their TOS explicitly states that they will preserve data: "We may access, preserve and share your information in response to a legal request"