On December 14th, Luna posted a group photo of f(x) with the caption, "It was joyful during the days when we were playful" and left the hashtags #f(x) #missyou #longforyou.
Many fans saw her post and commented, "We miss f(x) a lot too," "I miss you guys too, wish you guys would come back," and "f(x) is forever five."
Meanwhile, f(x) made their debut as a five-member girl group in 2009. They released many hit songs such as "Pinocchio," "Hot Summer," "Nu ABO," and "Four Walls." Their official group activities halted in 2016.