Depends on how long the prologue is. If it's less than a half a page long, then you'll probably want to post something else with it to give your readers more to chew on until you can post more installments.
Additionally, if your prologue has a completely different tone or takes place during a time period separate from the main storyline, usually, posting the first chapter in addition to the prologue will give the readers an idea of what they should expect from further chapters.