The Yo app is in the top 10 in the App Store in the USA plus it got 1 million dollars of investments.
It appears that the app was hacked by three students from college.
Firstly, cracking allows to recognize any phone number of the app user (hackers have already learnt phone number of app founder, and talked to him). Secondly, they can send "Yo" to any user in any quantity. Thirdly, cracking allows sending the push-notice to any user, with any text (guys decided not to do it).
Generally, hackers have already reported to the author about the problem and he has confirmed the fact of cracking. Now the developer tries to liquidate app vulnerability though it is precisely unclear what vulnerability can lead to the specified cracking.
Nevertheless, the author of the app promises to solve the problem "in some hours". The app was noted even by Elon Musk who had called it the best messenger (this message has already been rubbed, but the Internet remembers everything):
By the way, one more app user, named "ELONMUSK", created "a drill in a glass". He forced other users to believe that Elon Musk had sent messages to them. As a result there was a heap of joyful tweets “Elon Musk sent me Yo!"