Yes, it is. Usually, Facebook does this for some sites based on various factors. Putting the # at the end of the URL is a security measure. It prevents any 3rd parties that may be eavesdropping on the connection from appending additional information to the URL, since everything after the # is ignored by the server.
It shouldn't cause any adverse effects on your users or site, but if you think it's causing problems, let us know, and we'll gladly investigate if there's some other cause further.
Thanks,
Alex
# in url
