There’s some seriously bad behavior in Facebook that seems to kill IE8.  Leaving Facebook running overnight in IE results in the following memory utilization the next morning:

That’s 1.7 GB of memory consumed by IE8 just for the tab that’s running Facebook.  You can usually tell that things have gone wonky because IE starts showing the error icon in the lower left hand corner.  Unfortunately, shutting down IE doesn’t always work — sometimes the Facebook tab keeps spinning in the background (each tab is running in a separate process).  The only recourse at this point appears to be to kill the process (just the Facebook tab process — easy to find because it’s the iexplore.exe process consuming more memory than any other), which crashes the tab and causes a reload.  IE stays alive (thanks to the multiple-process tab model) and reloads Facebook in its original, low memory state. 

It would be easy to blame IE8 for this behavior, but I suspect that this is really a problem with Facebook’s client-side javascript code.