Cyberduck recently removed a particularly useful piece of information from their wiki regarding the sharing of bookmarks because it is no longer compatible with the sandboxed variant of Cyberduck available from the App Store. It is, however, still compatible with the Windows and OS X download available directly from its website.

To setup bookmark sharing between Cyberduck clients (works with both OS X or Windows), simply create a folder in your cloud sync folder and then point Cyberduck to it.

On OS X, open a Terminal and execute:

On Windows, press Super+R (Super is the key with the Windows logo on it) to open the "Run" dialog, and enter %APPDATA%. Next, open the Cyberduck.exe_Url_[some_garble]\[Version]\user.config file and modify the config file to add the new parameter:

edit 2021-04-30: Cyberduck has since upgraded to use MacOS's app containerization feature, so this hidden preference no longer exists. However, you can still sync bookmarks - you just need to symlink the "Bookmarks" folder that now lives under the app container path (see the Cyberduck FAQ) to an external directory, e.g. one in your Dropbox or Google Drive: