Clearing Dialer Storage data
If you clear the Dialer Storage data, it will delete all of your SMS/MMS messages. Deleting the cache didn't seem to matter. My phone did automatically delete my SMS/MMS messages once when my space was low for too long...
I am curious about deleting the Facebook and Facebook for HTC Sense data...each are at 22 MB, but I am tired of getting the "low on space" message every day.