Facebook SDK FQL: Getting profile URL?

I a开发者_开发知识库m using fql from facebook and querying user table to get profile_url

string facebook_info = Convert.ToString(FacebookAppExtensions.Query(app, "**SELECT 
                       profile_url from user where uid=" + facebook_ID**));
facebook_info = facebook_info.Substring(17);
char[] charsToTrim1 = { '}', ']', ' ', '"' };
facebook_info = facebook_info.TrimEnd(charsToTrim1);

The problem is that for some users the URL works while for other users, facebook says "UNKNOWN" page, is this a bug?

Thanks Behrouz

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no need to query: http://www.facebook.com/profile.php?id=UID

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I think as all data you can fetch this is controlled by the user too. If user doesn't let you fetch data about him, then you don't get that data via FQL.