Hi,
I guess it depends on the sort of monitoring you are after.
I use Cacti to graph traffic of the interfaces in the Cisco ASAs. Depending on the traffic flows you can usually work out which interfaces a large amount of data is traversing through. There are also specific plugins for the ASA to assist with the setup.
If its actual data flows you are wanting to trace then you may need to enable netflow and use a netflow collector tool. This allows you to see the actual data flows that are going through the firewall.
The below page has some links regarding Netflow if your interested
http://www.cisco.com/en/US/products/ps6120/products_installation_and_configuration_guides_list.html
Hope it helps
David