R.J.
Extremely often. It comes in waves. It will do it all the durn time for awhile, and then not at all.
So much so that I select and copy MOST of my long responses before I hit send.
The problem is when it's been working well for awhile that I get lazy, and forget to copy. Then *poof* gone. Thwibbt. There are a lot of things I've responded on that vanish and I just don't have the time to rewrite. It's part of why I get SO upset when whole posts and all the responses get reported for specious reasons. We go through spates of those, as well. When someone reports all the religious posts, or all the liberal posts, or all the conservative posts, or BF'ing, or spanking, or, or, or. We'll get someone newish on who just reports everything they don't agree with. And *poof* everyone's posts get erased.
Regardless of the cause... it's vexing.