I have tested this in your quick start file as well as my own fresh install and i get the same problem. The main menu works perfectly but as soon as i login to jomsocial (or Joomla user) the main menu is just white with no links. All other menus are fine, its just the main menu and as soon as i log out the menu reappears.
I cant check the main menu permissions as they are not set in module manager but in the template config.
I am using the most up to date Jomsocial,Party Freak template and Joomla 1.7.3.
Any chance of a little guidance on this matter?