From Facebook Developer Wiki

Contents 1 Description 2 Parameters 3 Response 4 Notes

Description

This method returns the number of friends of the current user who have accounts on your site, but have not yet connected their accounts. Also see fb:unconnected-friends-count. Note that this number is determined using the information passed via connect.registerUsers. If you have not previously called that function, this method will always return 0.

You can use the response from this call to determine whether or not to display a link allowing the user to invite their friends to connect as well.

Parameters

Required	Name	Type	Description
required	api_key	string	The application key associated with the calling application. If you specify the API key in your client, you don't need to pass it with every call.
	session_key	string	The session key of the logged in user. The session key is automatically included by our PHP client.
	sig	string	An MD5 hash of the current request and your secret key, as described in the How Facebook Authenticates Your Application. Facebook computes the signature for you automatically.
	v	string	This must be set to 1.0 to use this version of the API. If you specify the version in your client, you don't need to pass it with every call.
optional	format	string	The desired response format, which can be either XML or JSON. (Default value is XML.)
	callback	string	Name of a function to call. This is primarily to enable cross-domain JavaScript requests using the <script> tag, also known as JSONP, and works with both the XML and JSON formats. The function will be called with the response passed as the parameter.

Response

This method returns an int that indicates the number of users who have not yet connected their accounts.

Notes

You can call this method using a session secret, and not the application secret (for example, for a Facebook Connect site or desktop application).