Telegram is using regional clusters, probably and fully replicating everything. And using one central sign on point. You autenciate, then get a process on their server, that feeds you data from their internal network. Stream the channel feeds, buffers group messages, notifications, person to person messages, etc
And telegram message volume is actually very low, its just for humans. While Skycoin's system will be doing machine to machine communication and have significantly more data (but we do not have to retain the data forever).