To get an indication how bad this is, the default Steam account page was showing other people's accounts.
EDIT: Steam fully down for now. If you had a Steam Account, I recommend checking email/credit card on any linked accounts. (and, as always, sign up for 2FA if you haven't!)
EDIT2: Steam Community Moderator response (not linked):
- No, Steam is not hacked
- Creditcard info and phone numbers are, as required by law, censored and not visible to users