There are some ways to do this, with limitations. The most straightforward way (but not efficient for many users) is to edit the user's Profile and update the value for the Start Page property. Sites is not one of the options in my testing, but you get pages like Home, Gallery, Content, Map, etc.
Manage Profile and Settings
There is a way to loop through users in Python and update each user's property. That will take some time to write the code and if new users are created regularly, you'd have to run the script periodically.
A Product Manager had said there was going to be a way to set the default value for this, but I don't see it in 10.8.1 or the documentation for 10.9.x.
