In my case it's a mix. I either take them down to one of the local charity shop, or pass them onto friends and family. I can't remember ever chucking one out after one read. (I have occasionally chucked out really old ones after reading...stuff I bought tenth hand from a charity shop or passed to me from friends/ family where I thought condition was terrible.)
That..passing them on to a charity shop or to a friend..is the norm for people I know well.
There's also a fairly strong market..of course..in buying pretty old paperbacks on platforms like Ebay. I have got some stuff no longer in print from favourite authors such as Victor Canning at very low prices.