Yo's logo is also as minimalist as possible, simply a shade of purple. Its security was weak enough that a group of college students hacked it. That led its co-founder to write a post, headlined: "We were lucky enough to get hacked."

Yo does not appear to have a lot going for it. Yet somehow it caught on. The app's Twitter feed reported this week that it has more than one million users. Any start-up employees who have toiled away for months on a little-known app can be excused for bashing their heads against a wall in frustration.

Yo appears likely to be a fad. As tech blogger Robert Scoble put it, Yo is the pet rock of 2014. On Sunday, it began falling down the list of the most downloaded free apps. It still closed the day in a respectable spot, No9 overall.

Whether Yo lasts, its success is a reminder that the digital world remains a place for entertainment, conversation and single-servings.

When apps or websites are intensely narrow in their focus - and deliver a single serving - success comes easier: users know exactly what they are going to a site for.

The app Make it Rain, in which users swipe their screens to accumulate dollars, succeeded for similar reasons to Yo. It reportedly was generating US$50,000 a day earlier this year. It's a simple, silly and dumb app.

There's something humorous about an app that is incredibly focused on something very small. Anything that is funny is worth talking about and sharing, which helped Yo gain that viral traction.

But some technology industry moguls are taking Yo seriously.

Entrepreneur Marc Andreessen wrote on Twitter that people who make light of it are missing the point; that Yo is "an instance of 'one-bit communication' - a message with no content other than the fact that it exists. Yes or no. Yo or no yo."

He chided the naysayers for being biased. He wrote: "The hilarity around Yo includes two problematic biases: bias that one-bit comm isn't useful, and bias that all markets are like the US."

The trend of digital single servings can be seen in Facebook's strategy. It has recently been unbundling its services into a handful of narrowly focused apps, such as Messenger, Paper and Slingshot.

If there's a lesson in Yo, it may be this: apps that do something fun that no one else offers - no matter how dumb the concept - are likely to succeed.

Keeping it simple, and stupid can - for better or worse - be a winning strategy.

Rave reviews for 'Yo' app

The simplicity and absurdity of the one-message app, Yo, have helped rocket it to popularity, and led to hundreds of silly reviews in Apple's App Store, where Yo is raved about as everything from a cure for cancer to mankind's greatest creation. Here is a sampling of those five-star reviews for Yo:

"The phrase 'world-changing genius and perfection' gets thrown around a lot these days, but here's a case where the term is clearly an understatement." — Wordophile

"The feelings I have for this app are indescribable. Is this love? Fascination? An obsession?" — Jamiejsnpo

"The printing press. The car. The plane. Yo. This app will go down in history as the greatest innovation of the 21st century." — LinkOrlando

"It's gotten to the point where I exclusively communicate with the outside world via this app. Yo has torn apart every fabric of my being and rebuilt my life from the ground up; I am twice the man I was and living the dream." — John C Mayer

"You bet I'll deactivate my [Facebook], Twitter, Snapchat, email and credit cards for this app. Because it does it all." — CC234

"Speaking is now unnecessary and obsolete- Yo is all I'll ever need." — Yo.guys

"This app has changed my life. Before I downloaded Yo, my dog and I could not carry out conversations ... Since downloading Yo, Petey and I talk all day and night." — Hussgert2

"We no longer need intellectual discussion. We no longer need language. This is the next stage of human evolution." — Adam9812