[dotancohen]

Not OP, but such ideas usually stem from a misunderstanding of root cause. Facebook inaccessibility likely exposed poor assumptions or other flaws in e.g. "smart" devices or workflows. Those poor assumptions or other flaws are likely what got OP locked out of his apartment when Facebook went down, not Facebook itself going down.

[tyingq]

An apartment-complex entry app with Facebook login integration seems possible to me. That would be direct root cause.

https://developers.facebook.com/docs/facebook-login/

[dotancohen]

No, that would not be direct root cause. Direct root cause would be designing and implementing an apartment-complex entry app which depends on a working internet connection, battery, and network route to a single point of failure.