Playing a little more with Riffs - it looks like they have imported a lot of Amazon's catalogue. If you do a search, you are likely to find a lot of items that no human has entered. These items all have direct links to Amazon's product pages. The weird thing is that they are still associated with 'common tags'. These tags look very machine created. Are they pulled from Amazon somehow? If so, it sort of breaks the whole point of folksonomic tagging (unless you consider Amazon to be one big intelligent agent - which would be cool!)
For me, this somehow breaks the whole image of the human created, community content that Riffs wants us to think it is all about. Is it just another Amazon affiliate?