Why bother writing a whole message when you can say everything with just one word: "Yo"?
That's the philosophy of Yo, a "single-tap, zero communication tool" that lets you show someone you're thinking of them by sending them a "Yo" (much like a Facebook "poke").
The app is ridiculously simple to use. Once you've signed up you can add friends based on their username, or send them an invite. Just tap on a person's name in your list of contacts and they will be sent a notification that says "Yo".