If someone totally took over my Facebook account I'd be minorly annoyed and that's about it. Unless you're putting your social security number into your Diaspora account, who cares?
If I meant someone real harm and had access to their Facebook account, I could cause them a lot of trouble. I could send messages to their friends asking for unreasonable favours, starting arguments or causing other offence, I could embarrass them in front of their family. I could stalk them in real life. Look what happened when 4chan got hold of the passwords to a bunch of Christian student Facebook accounts:
Your online reputation is valuable.