The freedom to run the program means the freedom for any kind of person or organization to use it on any kind of computer system, for any kind of overall job and purpose, without being required to communicate about it with the developer or any other specific entity. In this freedom, it is the user's purpose that matters, not the developer's purpose; you as a user are free to run the program for your purposes, and if you distribute it to someone else, she is then free to run it for her purposes, but you are not entitled to impose your purposes on her.
Our favorite messaging service takes security seriously—it's Snowden-approved!—using its own open-source protocol to do end-to-end encryption, even on voice calls. It's not as much fun as some of the others, but still supports sending photos and video, plus group messaging. On Android, Signal can completely replace the SMS texting app; on iPhone you need to get other users to download Signal or you can't talk to them.
Proprietary software uses restrictive software licences or EULAs and usually does not provide users with the source code. Users are thus legally or technically prevented from changing the software, and this results on reliance on the publisher to provide updates, help, and support. (See also vendor lock-in and abandonware). Users often may not reverse engineer, modify, or redistribute proprietary software. Beyond copyright law, contracts and lack of source code; there could be additional shenanigans keeping users from exercising freedom over a piece of software, such as software patents and digital rights management (more specifically, tivoization).
Facebook Messenger is built right into the interface on Facebook on the Web, so you can use it without doing anything special. But downloading the mobile clients or using the dedicated Web interface at Messenger.com opens up lots of other options, from voice and video calls, to SMS texting support, to use of stickers, to, perhaps the best part, chatbots that will talk to you and provide info. Like its brother, WhatsApp, it also has a billion users.
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