The old potting mixes I have usually go into the compost bin with the other waste material from the garden and kitchen. On occasion we have dumped them into one or another of the planting beds to add some organic matter there. Most all of these potting mixes will be peat moss so additional organic matter will be needed to supply the nutrients that peat moss lacks.

I too put some of it in the compost bins; some of it I spread out on tarps, expose to the sun for a few days, then bag for future use in containers, to be mixed with fresh materials.

I too dump mine in the compost bin. It seems to give it a burst of heat a few days later, which is always nice.

I spread mine over the perennial beds.

I reuse/recycle/rejuvenate mine year after year. The issue is maintaining, year to year, enough structure in the potting mix so that there is good water and air circulation. Its a bit counter-intuitive - you don't need to worry so much about the organic matter, but you do need to worry that there is enough perlite, dime-sized bits of bark, stones, etc so that the air gets into the mix.

There are several threads over on the container gardening forum that address recycling container soils.

Here is a link that might be useful: link to container forum

I have a big group of mock honeysuckle in front of the compost operation and I dump the potting soil under there. In fact, a couple weeks ago, I went for the cheapest of the cheap potting soil and found it was mostly clay dirt, with a little peat and vermiculite. Not caring about returning it for the $3 it cost me, I dumped that whole bag under the bushes.

I reuse mine. When I worked in a nursery we always reused it. The only problem is weed seeds. If you can sterilize your pots with bleach and rinse them well it should be ok.

I dump ours under our two Dogwood bushes and
a Viburnum bush...

I reuse mine, too. I have been mixing my own for some time now, so when I make it, I just empty old pots into the mix.