This morning Dropbox posted a to communicate its users that the popular cloud storage service was not hacked:

Recent news articles claiming that Dropbox was hacked aren’t true. Your stuff is safe. The usernames and passwords referenced in these articles were stolen from unrelated services, not Dropbox. Attackers then used these stolen credentials to try to log in to sites across the internet, including Dropbox. 

Always use different passwords across services, and enable two-factor authentication.