:: WHAT'S NOT TO LIKE?
When a friend posts that his father has died, or a cousin gets frustrated with her morning commute, hitting "like" might seem insensitive. Users have long requested a "dislike" button, but that was deemed too negative and problematic.
Facebook chose to offer more nuanced reactions - "love," ''haha," ''wow," ''sad" and "angry" - alongside "like" - to give users "greater control over their expressivity," said Julie Zhuo, Facebook's product design director.
:: WHY THESE CHOICES
Facebook went through comments on friends' posts, as well as emoji-like stickers people were using. It chose the most common ones and tested those. Facebook considered dozens of reactions - but offering them all would have been confusing.
Facebook ultimately chose these six reactions for their universal appeal - something that could be understood around the world. Even a generic happy face "was a little bit ambiguous and harder for people to understand," Ms Zhuo said.
Each reaction comes with an animated emoji, such as the thumbs up for "like" and a heart for "love". These emojis will look the same around the world, but phrases such as "love" will be translated.
:: "LIKE" STILL TAKES CENTRE STAGE
Ms Zhuo said people click on "like" more than a billion times a day, so "we didn't want to make that any harder". It's still the go-to reaction for most posts. But Ms Zhuo says in the countries tested, people used the alternatives more frequently over time.
:: HOW TO GET STARTED
The rollout is expected to take a few days to complete. Users will get the feature automatically on web browsers, but an update to the app will be needed on iPhones and Android devices (no word yet on Windows and BlackBerry).
Facebook already shows how many people like a post and lets you tap or click on the count for a list of people. With Reactions, you see how many people have reacted in some way, along with the top three reactions, such as "love" followed by "haha" and "wow". You can get breakdowns for each reaction - the total and specific people.
:: A HAPPY BIAS?
Facebook has a complex formula for deciding which of your friends' posts are more prominent. Ones that get a lot of likes, for instance, will tend to show up higher. Now, posts marked "angry" or "wow" will bump up, too.
But Facebook wants to show what it thinks you're most interested in - and that might ultimately mean mostly happy posts, rather than ones that evoke sadness or anger. Ms Zhuo said Facebook will tweak its formulas based on how people respond.
:: EXPRESS THAT ANGER
These alternative reactions are for all posts, including those from groups and brands. A company will not be able to block the ability to mark its posts with anger.
:: IT TOOK A YEAR TO DEVELOP
Why so long? Besides deciding on how many and which specific reactions to offer, Facebook needed to figure out the right way for people to discover and use it. For instance, a menu might have been harder to find, while offering all six buttons up front might have made it harder to just quickly "like" a post and move on. Ms Zhuo said chief executive Mark Zuckerberg pushed for the long-press method as a balance.
The feature is expected to evolve over time, and Facebook may add or change choices based on feedback.