Swiss startup Threema probably didn’t expect this. In 24 hours, the startup has doubled its user base, according to Süddeutsche. It is now sitting at the top of the paid App Store chart in Germany. Interestingly, Threema’s key feature is its true end-to-end encryption — German users probably don’t want to use a Facebook-owned app anymore.

“Unlike other popular messaging apps (including those claiming to use encryption), even we as the server operator have absolutely no way to read your messages,” the website says.

While WhatsApp promises a great level of security, the startup faced security holes in the past. And of course, it is now part of Facebook after the $19 billion acquisition. Facebook is an advertising-based company, after all.

Facebook promised that WhatsApp wouldn’t change. It would remain an independent entity without an advertising-based business model. But apparently, these promises are not enough for German users.

WhatsApp currently has around 30 million users in Germany. It’s the indisputable leader. That’s why it’s interesting to see that a massive user base is ready to jump ship and switch to a brand-new service.

Threema only has three employees and has been overwhelmed by support requests. The app looks and acts a lot like WhatsApp, except that the startup’s server admins can’t even see the messages because they don’t have the encryption keys.

Thilo Weichert, data protection commissioner for the German Land of Schleswig-Holstein, said that the WhatsApp acquisition could create data-protection issues. “Facebook sees everything. And WhatsApp also sees everything,” he said to SHZ. According to Weichert, you should be careful with the services you are using. And he’s not the only one who thinks that — among the 200,000 new Threema users who paid $1.99, 80 percent of those live in Germany.

As a reminder, Angela Merkel’s phone has been tapped for years by the U.S. intelligence agencies. It’s a very pragmatic example of a privacy breach. It probably resonated with Germans. Now, WhatsApp will have to make sure that its users still feel safe sending WhatsApp messages. It will have to prove that privacy is still one of the company’s top priorities, Facebook or not.