No longer able to copy file link in Dropbox after upgrading Mac to Yosemite
Make sure that you are using a 2.10.4x build of Dropbox. The current latest stable version is 2.10.45 and can be found at 

https://forums.dropbox.com

Dropbox's Finder integration in Yosemite is very different because there is now an official way to have Dropbox integrate with Finder, rather than Dropbox having to inject its code into Finder.

(See the “Finder Sync Extensions” section of OS X 10.10 Yosemite: The John Siracusa Review for more details.)