In my experience, it replaced Craigslist. Now that Craigslist charges for listing items, why not use Facebook where listing items is free and you can at least vet the persons profile before going to meet up with them?
In my area at least, nextdoor.com is really good for posting and finding items as well. Plus you know they're nearby and you're not having to drive super far to get an item. I found some awesome free patio chairs through nextdoor.