I'm 99% sure those apps send those updates directly back and forth between users. Riot can't do that, because it opens up security holes where you can get someone's IP address, which then allows you to do things like DDOS them during a match.
Also, those services don't also need to be serving data for millions of ongoing realtime strategy games. Your message takes half a second longer to arrive on WhatsApp because their network is overloaded? Whatever, most people wouldn't even notice. The packet saying you activated Flash takes half a second longer to arrive? You just died and lost your match.
And sending 1 message is a bigger update than sending an update that your friend is typing...
It's FAR more efficient to send large packets than small ones. Often the limiting factor for routers these days is dealing with a bajillion tiny little packets that contain almost no information. In fact, that is what most DDOS attacks do, overload a company's routers with a constant stream of useless packets they have to deal with. Having to literally send millions of these back and forth per second could be a huge strain on their network infrastructure. This is the same reason (I'm pretty sure) that they don't offer a "ping meter" in their client like people keep asking for. They'd basically be DDOSing themselves.