Yes, compost is always in the process of breaking down... it can turn into a sort of dust when it sits long enough in a container, eventually getting flushed out the bottom. If you find that there's no soil left in the pots where your plants are growing, consider adding ingredients to your soil mix with greater longevity. The barky type of compost takes longer to break down. Peat and coir (coconut fiber) both have a pretty long lifespan in containers. I recommend the latter if you can find it, because it's kinder to the environment and rewets faster when it goes completely dry. Also, aggregate like pumice or perlite has an essentially infinite lifespan in pots because it does not break down or compact. And you will find your soil is kinder to your plants for longer if you start with a gritty mix (filter out and get rid of the fines). More oxygen by the roots, less compaction over time.
My mix for growing succulents consists of 25% potting soil (mostly compost), 25% cocofiber, and 50% pumice. When I revisit plants that have been growing in it for years, I usually see the color has changed from a dark brown (the color of compost) to lighter brown (the color of coco fiber), owing to the release of compost from the container over time.
