I was going to edit this but then realized the information I was going to add from wikipedia needed a citation but here is the link and my copy paste skills.
"Psilocybin is rapidly dephosphorylated in the body to psilocin which then acts as a partial agonist at the 5-HT2A serotonin receptor in the brain where it mimics the effects of serotonin (5-HT). Psilocin is an 5-HT1A and 5-HT2A/2C agonist"
which means psilocin is also being taken up by theese other receptor sites and correcting 5-HT2 to 5-HT2a.
http://en.wikipedia.org/wiki/Psilocybin