I am trying to write an app that has a messaging feature in it. It needs to have the ability to handle pictures and other types of generic data files. It also should be push and almost instant. Also, it would be great if there was a way for it to be cross-platform, especially between iOS and Android. I am not very familiar with any of the current protocols for handling push messaging. One idea I had was XMPP, but I don't think that is how very many apps do it. What protocols are used by apps like Facebook Messenger? I know that some sort of REST HTTP framework could be used, is that the best way?
To my knowledge, XMPP is used in many applications in GGPlay such as Wechat, GOSMS, Whatsapp,Cubie,LINE,Gtalk... is also use XMPP. GTalk use its own implementation based on XMPP. Others use Smack. For Android you can use A-Smack which not supported longtime ago. Another option is Beem which is current develop and improved. For iOS use xmppframework library. You can find libraries for others languages here