Private messaging apps are having a moment after Twitter and Facebook cracked down on violence and conspiracy theories in response to the Capitol riots.

Private messaging services Signal and Telegram were the No. 1 and No. 2 most downloaded free apps in Apple’s App Store and the Google Play Store on January 13. The shift is believed to be driven by Trump supporters seeking alternatives to mainstream social media, as way to punish the companies for banning Trump over his incitement of violence.

Signal and Telegram both offer encrypted messaging, which means that they scramble communications between users to protect the data from being analyzed by third parties. The encryption makes it difficult for law enforcement to monitor users.

Indeed, some news reports have said that pro-Trump fringe groups are using Telegram to organize more protests expressly to keep their conversations private from police.

Besides letting people send text messages and chats, Signal and Telegram also provide audio calling services that are difficult, if not impossible, for others to access.

Facebook's WhatsApp service, which also offers private messaging, isn't adding users as quickly as its rivals (on January 13, it ranked 16th in Apple's App Store and 19th in the Google Play Store). Some users are concerned about WhatsApp starting to share some data with its parent company.

Earlier this week, two other social media services that have become havens for Trump supporters, Gab and MeWe, said they had added hundreds of thousands of users in the past week. In addition to benefiting from the same shift by conservatives from Twitter and Facebook, the sites gained users who had been left stranded by the closing of Parler, another Trump-friendly site that failed to police violent posts on its service.

Here’s some more details about free messaging apps.

What is Signal?

Signal, maintained by the non-profit Signal Foundation, has long been a favorite method of communication for by privacy advocates and journalists. Brian Action, a co-founder of the Facebook-owned WhatsApp service, is the executive chairman of the Signal Foundation.

What is Telegram?

Unlike Signal, Telegram users must activate the encryption feature before using it. The service can also accommodate group messages of up to 200,000 people compared to a maximum of 1,000 on Signal. Pavel Durov, a Russian entrepreneur who was Fortune’s 40 under 40 list in 2018, is Telegram's CEO.