Deserialise json in apex

There is one good tool available. JSON2Apex

Using that you can generate a class and using that wrapper class you can get the data.

//
// Generated by JSON2Apex http://json2apex.herokuapp.com/
//

public class JSON2Apex {

    public Posts posts;
    public String id;

    public class Paging {
        public String previous;
        public String next;
    }

    public class Posts {
        public List<Data> data;
        public Paging paging;
    }

    public class Data {
        public String message;
        public String created_time;
        public String id;
        public String story;
    }


    public static JSON2Apex parse(String json) {
        return (JSON2Apex) System.JSON.deserialize(json, JSON2Apex.class);
    }
}

Now parse your JSON in parse method and it will give you JSON2Apex instance and from that you can easily get data (message or any other value) from this.

You could use a JSONParser and iterate through the Tokens plucking out the "message" values...

list<string> messageList = new list<String>();
JSONParser parser = JSON.createParser(JSONContent);
// Advance to the next token.
while (parser.nextToken() != null) {
    System.debug('Current token: ' +  parser.getCurrentName());
    if(parser.getCurrentName() == 'message'){
        parser.nextToken();    //We want the value, so advance to the next token.
        messageList.add(parser.getText());
    }
}
system.debug(messageList);

This pattern, while not terribly efficient, provides a loosely typed solution that won't require code changes if the structure of the input JSON changes.