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:
defaults write ch.sudo.cyberduck application.support.path ~/Dropbox/Cyberduck
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:
<setting name="CdSettings" serializeAs="Xml">
<setting name="application.support.path" value="C:\Users\yourname\Dropbox\Cyberduck" />