Issue saving data accross multiple platforms

Hi, im creating a game that is intended to run on Windows Phone 8, Android and iOS. So far I have used a file storage method using binary serialization.

Below is my code for saving and loading:

 public void Save()
     {
         //saves data out to a file.
         BinaryFormatter bf = new BinaryFormatter ();
         FileStream file = File.Create (Application.persistentDataPath + "/playerinfo.dat");
 
 
         PlayerData data = new PlayerData ();
 
         data._highscore = _highscore;
 
 
         bf.Serialize (file, data);
         file.Close ();
     }
 
     public void Load()
     {
         if(File.Exists (Application.persistentDataPath + "/playerinfo.dat"))
         {
             BinaryFormatter bf = new BinaryFormatter ();
             FileStream file = File.Open (Application.persistentDataPath + "/playerinfo.dat", FileMode.Open);
             PlayerData data = (PlayerData)bf.Deserialize (file);
             file.Close ();
 
             _highscore = data._highscore;
     
 
 
         }
     }

This seems to work fine on the PC but when I tried to export it to Windows phone 8, I got an error:

Error building Player: Exception: Error: type System.Runtime.Serialization.Formatters.Binary.BinaryFormatter doesn't exist in target framework. It is referenced from Assembly-CSharp.dll at System.Void dataManager::Save().

I looked into this and it seems that the serialisation function is not compatible with WP8, so my question is: How would I got about saving game data accross all of the platforms? Thanks for any help!