A reaction status is a Facebook status
posted purely because the person posting it is aware that it will have a reaction (a large number of comments/likes)
A common 'reaction status' is seen after a popular TV show, when people post funny lines from the show knowing that people will be logging into Facebook shortly afterwards.
"How good is half term?" (obviously half term is good, but why would you say this for any other reason than to get a reaction? Reaction Status.)
"Remember when everyone used to have a Nokia 3310?" (The person posting this knows everyone had a 3310. They said so in their post. So why ask? Reaction Status.)
*any line from The Inbetweeners
* (People posting these know large numbers of people watch The Inbetweeners, and often actually race to be the first to post a funny line, in order to secure the most comments/likes. Reaction Status.)