Normally, the app is able to import nearly all of your active friends. Nevertheless, sometimes some friends seem to be missing. The reasons for this can be following:

• the friend has deactivated his profile - those will be count in your list of facebook friends, but those profiles can’t be used in CoSy
• the friend does not have any public visible profile image
• the friend may have disabled the ability for his profile to be searchable

Additionally, the import may stop at any time because of a timeout, an unexpected error or similar. In this case, simply try to repeat the import, the app takes care to not import friends twice.

You can find some related informations here (slow import) as well that explains why the import sometimes may stop.