[Facebook Unity SDK] Issues while storing multiple FB API calls results in array.

Hello,

I am trying to retrieve 25 non-friends facebook users first names from Facebook but I have issues while using indexes in callback :

Code (CSharp):

1.  
2. m_leaderboard = new LeaderboardData[25];
3. for (int i = 0; i < 25; i++)
4. {
5.           yield return new WaitForSeconds(0.5f);
6.  
7.           // We just have to load first names
8.           FB.API("/" + m_leaderboard[i].fuid + "?fields=first_name", Facebook.Unity.HttpMethod.GET,
9.               delegate (IGraphResult result)
10.               {
11.                   if (result.Error != null) return;
12.                   Debug.LogWarning("Index { " + i + " }");
13.                   m_leaderboard[i].first_name = result.ResultDictionary["first_name"] as String;        
14.               }                                  
15.           );

where m_leaderboard is just a struct tab storing leaderboard infos :

Code (CSharp):

1.  
2. public struct LeaderboardData
3.        {
4.                public long fuid;
5.                public string first_name;
6.                public Texture image;
7.                public int score;
8.                public int rank;
9.        };
10.  
11.        public LeaderboardData[] m_leaderboard;
12.  

The problem is indexes are wrong in callback (timing issues, some "i" are skipped), how can I chain several API calls and store results in array ? (I could probably do 25 API calls and index them without the for loop but ...).

Second problem is how can I trigger an event when all callback threads are terminated :
Actually, I work with a counter incremented in callback which fires an event when 25 values are received but it seems to me that it's not optimal. I was thinking of something like Thread.Join() in C and fire event when all callbacks are finished.

Thanks for your help,
mickee