I use Noobhub, it’s open source, very powerfull and needs only a very cheap server for thousands of concurrent players. It’s the fastest multiplayer server code out there. I use it for my realtime multiplayer game WordnRoll. But you need to dive a bit in coding javascript (node.js) and a bit of server management.
I implemented with noobhub highscores, realtime multiplay, account management, player lobby, random matchmaking, inviting friends, push messages (with Pushwoosh, I was to lazy to implement a node.js plugin for it) etc.
So it’s basically an implementation of node.js, a node.js auth plugin, mongodb and a cheap VPS Linux server. The nice extra thing is you can use it very easily for a website too, to give some extra player support, info, stats, account management etc.
The author of noobhub is very friendly (Overtorment) and there is some support on the Corona Noobhub thread.