How much of a song can i legally use?

For example, fixation occurs when a song is recorded in an audio file or when a musical work is annotated on a score or in a digital file. However, you can't just cut a 5-second clip from a popular song and use it in a Facebook ad, that's definitely not “fair use”, even if you're only using a very short clip. If you're looking for a song by a well-known artist, you'll need to contact their publisher and record label to negotiate the license.