Very little, as the advertising companies think it's quite inefficient to do on larger scale. The profiling of the demographics is pretty much nonexistent, so the cost of blindly sending hundreds of thousands of leaflets would be prohibitive.
There are other ways, though. Some local businesses (notably furniture, electronics and gardening) hire mailbox stuffers to go door to door. The regular houses don't get so much of it, because there's much more walking to do to reach all of them, so many stuffers concentrate on high rises in novogradnja, where they have dozens of mailboxes at the same place. Many such places, however, have already posted dire warnings against them on their doors. Perhaps there's some regulation on their side and the threat may not be empty.
A particularly dirty trick is when you get an ad slipped with your utility bill, specially when they aren't selling anything tangible, e.g. insurance. Many people simply start paying without ever seeing a contract.