You need to provide more information, but from what you have said so far,

do not use EJB for presentation, it is way too heavy as you say your system may interact with thousands of users concurrently. User Servlets instead. for heavy accessed presentations, Servlets and JSP is the best solutions recommended by Sun

For persistance, please note that Hibernate is light weight, but it does not support transactions, so you need to answer the question as how transactions are going to be supported?

if you provide more details regarding your object model, it would be more helpful for giving suggestion, sometimes it's better to use DAO and Transfer Objects with BMT usually if object model is very complex, if not and if transaction is a must go with CMT.

just consider the difference between Hibernate and Entity beans.