Is it possible use Dropbox as my OSX Time machine backup drive?
I am thinking of getting Dropbox Pro.
Dropbox themselves say it's not possible.
Time Machine uses a special way of "pointing" files to each other, which means that every snapshot of your system as available in Time Machine is not the same on the disk. For example, a file you created a month ago will not be duplicated in your current backup, but there's a "pointer" to the old file in another folder.
Dropbox however doesn't offer a way to represent these file links in their system. That means you can only upload actual files to their service, and not those links.
As a consequence, you can't recreate the file system layout of a Time Machine drive in the Dropbox when pulling it from another Mac or looking at it online, even though your Mac would appear to create the backup just fine.
If you want to spend money on online backup services that fit well with OS X, check: