The company did say the number of subscribers that will be forced to take new passwords but also maintain that it had taken the move pro-actively.

In their words via Motherboard:

“Our security teams are always watching out for new threats to our users. As part of these ongoing efforts, we learned about an old set of Dropbox user credentials (email addresses plus hashed and salted passwords) that we believe were obtained in 2012. Our analysis suggests that the credentials relate to an incident we disclosed around that time.”

The data that was stolen had details of users who had signed up to Dropbox around 2012 or earlier. If you signed to Dropbox around this time, then you need to change you login details like me even if you didn’t get an alert to do so by the company. 

You might also consider improving the level of you security on other platforms that share the same login details with you Dropbox.