A legal finding of fair use takes into account the following factors: The purpose of the use, The amount of the work to be used, The effect of the use on the market for or value of the original work, and The nature of the copyrighted work. There is no official definitive answer for whether a use can be considered fair, as every case must be judged on its own merits, but there are some types of use generally allowed under fair use, including criticism and commentary, parody, journalism, education, and research.
Intent has such a huge impact on legality. If it's something you found funny and you just want to share it with your friends, that's fine (as long as you aren't causing harm to the subject of the meme if they are a real person). If you're a big company and using the meme to advertise your product, be prepared for a lawsuit.