Mark Zuckerberg, CEO of Facebook, which owns WhatsApp. Photo: Reuters/Yuri Gripas

WhatsApp told users on January 6 they would have to agree to let Facebook and its subsidiaries collect WhatsApp data — including phone numbers and locations — before February 8 or lose access to the app. WhatsApp has since clarified that this affects users only outside the European Union and the UK and said that the change "does not affect the privacy of your messages with friends or family in any way."

Data from app-analytics firm Sensor Tower shows Signal, a rival encrypted messaging service, saw an enormous surge in user numbers following WhatsApp's announcement.

"From January 6 to January 10, Signal saw approximately 7.5 million installs globally from across the App Store and Google Play," a Sensor Tower representative told Insider.

This represented a 4,200% increase from the previous week.

On Wednesday, the app topped both Google and Apple's app stores in the US, according to data from App Annie, the analytics company. Fox News reported the app has been top of both stores since Monday.

Signal received significant publicity following WhatsApp's announcement, with public figures including Elon Musk and Edward Snowden endorsing the app. Musk tweeted "use Signal," which had the unexpected side effect of sending stock for an unrelated company, Signal Advance, soaring by 11,700%.

The encrypted-messaging service Telegram saw even more users flock to its service. Between January 6 and January 10, it amassed 9 million new users, up 91% from the previous week, and is now second in both Google and Apple's app stores.

For both apps, the biggest growth market was India. Signal saw 2.3 million installs in India — more than 30% of its total new installs. India accounted for 1.5 million of Telegram's installs, or 16% of all installs.

For Signal, the second-biggest market was the US, where users installed it about 1 million times.