In Esperanto, the word knuflo is mainly used by young people, who participate in local meetings, around the dutch-speaking regions of Flanders and the Netherlands. It is almost not used in international context, like on the Internet ("knuflo" esperanto gives less than 12 results with Google), outside of groups that are community specific. Outside of these groups, the word is not understood.
Knuflo is like hugging, but longer, with more parts of the body. It is like cuddling, but it is hard to describe. Users say you have to receive a knuflo to understand it, and you have to experience it to understand why the word is needed in the language.
So here, you have three factors. People sharing the same factor tend to speak together. When people speak mainly within their own community, slang appear.
Age factor: Young people coin new words for the new environment, and for concepts that are felt important for the social group. It is exactly the same in Esperanto. But there is no clear barrier. Here is a footage of an older person using the word "knuflo".
Location factor: Not all Esperanto-speakers participate regularly in Esperanto events, in fact only a minority do. There is an event-slang, with words such as "gufujo", "stelo (mono)", etc. Esperanto events are often attended by people from neighboring countries. This way local slang is shared among people of different countries and age. Because of this, you can sometimes hear "knuflo" in French-speaking Belgium and Germany (near the Netherlands). The word is also shared in huge international youth events like JES and IJK, where a consequent number of participants come from the knuflo-region.
Language factor: "knuflo" comes directly from the Dutch verb "knuffelen". Dutch-speaking beginners who are not aware of similar words like "brakumo" "kareso" "dorlotado" are more likely to remember and understand "knuflo". This kind of beginner-slang tend to not go outside the local club, but "knuflo" is an odd exception. Maybe because some local experienced speakers think it brings new meaning.