It was pretty obvious the dropbox hack was real several years ago, because lots of spam mail started arriving at my dropbox-unique email almost immediately after the breach. I changed my email to another unique address quickly back then. Unique-per-service email addresses work pretty well as a canary for breaches. Just make sure there is more uniqueness than just the service name to such addresses, or someone could see your pattern and start spamming by guessing popular services.
On a side note, don't forget the time dropbox accepted ANY password during logins - http://www.cnet.com/news/dropbox-confirms-security-glitch-no...