Has 'ought to' fallen out of use? No, it hasn't. It's used in everyday speech. You're lazing around in the pub with your friends during your lunch hour, and as work beckons, the following exchange might occur: "Another pint, mate?" "Nah. It's 2 o'clock. I ought to be getting back."

I use it sometimes, but not so frequently. Most people say "oughta" instead of ought to. It is slang. Like "you've got to" is often changed to "you gotta".

For me, definitely still in use.

Yes, "ought" has its purpose. I wouldn't say it has fallen out of use. In my opinion the negative "oughn't" is probably becoming rarer.