I'm trying to get the profile pic of the user of the game using this-
void MyPictureCallback(FBResult result) // store user profile pic
{
        if (FB.IsLoggedIn)
        {
            WWW url = new WWW("http" + "://graph.facebook.com/" + FB.UserId + "/picture");
            Texture2D textFb2 = new Texture2D(128, 128, TextureFormat.ARGB32, false); //TextureFormat must be DXT5
            url.LoadImageIntoTexture(textFb2);
            profilePic.renderer.material.mainTexture = textFb2;
        }
But it isn't working. I am getting no errors.
The Facebook SDK for Unity complements Unity Technologies' cross-platform support, providing a pure-Unity write-once, run-everywhere experience across the key gaming platforms of WebGL, Unity Web Player, Android and iOS.
Jason Pietka's answer is OK but a bit old.
Today we us FB.API:
FB.API("me/picture?type=med", Facebook.HttpMethod.GET, GetPicture);
GetPicture is a callback method so:
private void GetPicture(FBResult result)
{
    if (result.Error == null)
    {          
        Image img = UIFBProfilePic.GetComponent<Image>();
        img.sprite = Sprite.Create(result.Texture, new Rect(0,0, 128, 128), new Vector2());         
    }
}
                        I fixed it with this-
WWW url = new WWW("https" + "://graph.facebook.com/" + userId + "/picture?type=large"); //+ "?access_token=" + FB.AccessToken);
            Texture2D textFb2 = new Texture2D(128, 128, TextureFormat.DXT1, false); //TextureFormat must be DXT5
            yield return url;
            profilePic.renderer.material.mainTexture = textFb2;
            url.LoadImageIntoTexture(textFb2);
            Debug.Log("Working");
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