From Facebook’s announcement:
The security of people’s accounts is paramount at Facebook, which is why we constantly monitor for potentially malicious activity and offer many options to proactively secure your account. Starting today, we will notify you if we believe your account has been targeted or compromised by an attacker suspected of working on behalf of a nation-state. This is what the notification looks like on the desktop version of the Facebook website:
Read more of Alex Stamos’s announcement at Facebook.