Can a hacker randomly hack someone's Facebook account, perhabs because the victim has a weaker password (eg a short word and 1 symbol), even if he doesn't know the victim's email or mobile number or the victim itself?
Unfortunately, if the hacker has the Facebook username, they may try several passwords in order to steal your account.
A hacker may obtain your credentials by using phishing pages or keyloggers that records everything you type on your device. Never click on links from suspicious emails and only visit www.facebook.com or m.facebook.com to log in. Also use antivirus software on your device and perform regular scans.
To keep your account secure, use a strong password combining upper and lower case letters, numbers and symbols. You can also enable 2-step authentication to add extra protection. For additional advices, you can visit this help page in the Facebook Help Center.