Oracle Enterprise Manager 11g: How to see statistics of Interfaces usage?
If you are talking about Listener statistics on Oracle Enterprise Manager then-

1) Click on Listener name on Home screen of OEM.
 

2) Then Click on 'Established Connections per minute' under 'State' section.


3) Now you have got the page with some statistics of listeners.


For your last question "What kind of actions are mostly done?" , I think You need to use database auditing so that you can track the activities of users connecting using the listener.