Would be better if the PNs where grouped by user with ajax support (just scroll down to get older messages) like Facebook...
It is "already possible" (in some way) by adding new directories to separate manually in groups but it doesn't solve the main problem I think.
you have 2500 old PMs in your PM box and wanna read "that one" received (or sended) in a date you forgot to a certain user that contains some content that you remember about in a non precise way.
It is possible to find such PM in few seconds without opening a bunch of PM to see their content just only if you have a search box
that allows you to search inside the PM body. If you don't have a search box that allows that, you have to open every PM to find what you are looking for.
It is about the same when you are looking for some topic and you use the relative search box.
I'm I right?
Another thing: you are right about Facebook that allows to group PM by user. But If you have archived 2-3 years of PM and you need to search out "that PM" of "that topic" but you don't remember exactly neither the date nor the user you were talking with but you remember that you talked about a certain thing? What do you do? Facebook doesn't allow (yet) to do an "inside search" of this type. I think this is a bad thing of facebook that I hope will be solved in future updating, because time by time pms are growing up and time is going on and some forgetfulness may happens. Having an elastic tool like that may solve situations like that.
Any way you suggestion is a further improvement so +1 for that too and thank you for it