Well they rely wholy on the media and their interest in the case. How many abductions have you heard about in the past years that simply fell off the public agenda?
The reason this has stayed in the public eye is because there is still hope that Madeleine is still alive.
As long as the media, internet sites and caring people keep her in the public eye the there will always be interest.
As long as everyone in the know thinks they are doing their bit, then the word will spread very fast.
One file on the net can be downloaded by 2,000,000 people per day so thats a massive advertisement function, keeping her in the public interest is the key.