Given that a "Yo" usually follows a more detailed text message like "Text me when you're downstairs," I think the total finger-on-screen time would be higher to open the Yo app and search for the right contact.

I assume that Yo has another plan that they haven't gone public with yet, since "Company that just says Yo raises $1m" is more newsworthy than "Company to solve problem X raises $1m." They'll get people to download the app and then add the new functionality with an automatic update.

Because the app magically sends the Yos without having to be launched? Facebook messenger has the thumbs up as a single press, as the send button turns into that with null content.