Is someone else using my Facebook page.Why?

If you can still log in, do so on private wifi and change your password... As a warning never use a public wifi to access anything important especially any banking information, because anyone can join the same public wifi as you are on and from there they can use your cookies which stores things such as passwords and other account info.... This can also happen if your wifi is on while you walk by the local coffee shop "or any place with public wifi" and the device connects anytime during the area of connectivity... your best bet is to always have your wifi turned off and only connect to trusted sources with password protection.. and to answer your question why, people are miserable and only get enjoyment making others feel as bad or worse then they feel, i know its a ***** but its just how it is i hope this helps you in the future