reactions

 reactions  refer to the subset of responses/interactions with a post that are quicker, more impulsive, but still a conscious act, typically a simple UI gesture without writing any content, such as likes (reacji), reposts, bookmarks, or perhaps multiple simple UI gestures, such as selection a text range and posting a quotation of part of a post, or picking a person from a list to post an invitation as a response to seeing an event.

Chat
Chat tools that support emoji reactions to messages:

Open Source

 * Zulip
 * Element
 * Rocket.Chat

Silos

 * Slack - has had custom emoji support at least since 2015, but could they be added as metadata or just typed in messages?
 * Discord - Nitro required to add emojis from other servers

Facebook
Facebook supports:
 * reposts (as "Share")
 * simple bookmarks (as "Save post" or "Save link")
 * expanded like reacji: 👍❤️😆😮😢😠 (more: reacji)

Facebook also refers to their reacji as "reactions":



Productivity

 * GitHub: Allows selecting from about 7 different emoji, much like Facebook, and adding to issue and discussion comments
 * GitLab: Allows selecting any emoji

The Tapestries
On The Tapestries, reactions include Like, Agree, Disagree, and various emoticons and emoji like <3 and :zap:

Premium Emoji
Streaming platforms including Twitch and Facebook have emojis that can be added for subscribers

FAQ

 * asks "is a citation a reaction"?
 * undefined IMO no, since a citation usually implies considered inclusion and reference to support a specific point being made, rather than being impulsive.