I have created a field on the contact record titled Twitter User Name.  I am now trying to create a hyperlink formula field that will take the user name entered and add it to the Twitter URL and I can't get it to work.  It works great as a custom link but I want to be able to place the URL in the page layout and not in the custom link section.  Any suggestions?

I have tried both of these options

HYPERLINK("http://twitter.com/Contact.Twitter_User_Name__c", "Twitter Account" )

HYPERLINK("http://twitter.com/Twitter_User_Name__c", "Twitter Account" )