I just received this email from Dropbox. I have the free version, but only using 84.9 MB of 2 GB available. Email also included link back to Dropbox to upgrade to paid plans.
Is this a new method to get people to upgrade to paid plans?
If you don't mind less app integration, I'd suggest giving Cubby [1] a try.
It has most of the Dropbox features with 5 GB of free space and killer feature of making anything a cubby, just draf and drop any folder on desktop app and to sync it automatically.