Traditionally, music charts have been all about music sales. You sell records, you get in the charts.
The UK (and other) charts have undergone a number of changes over the past few years. As physical cd sales declined, the chart rules were modified so that MP3 sales on iTunes and other digital retailers were taken into account. This led to some interesting effects: popular album tracks started entering the single charts; and obscure songs that had not been released as physical singles started to make surprise appearances, due to positive word of mouth and "grassroots" followings of underground acts.
The charts may be about to undergo another profound change. This is because I think that conventional sales - upon which charts are built - are on the way out. It seems increasingly likely (thanks to services such as Spotify) that people soon won't buy MP3s or cds any more, but will stream whatever songs they like over the internet (either on a subscription basis or by agreeing to hear adverts between tracks).
If this happens, the focus of charts is inevitably going to switch away from sales to the number of plays of particular songs. To a certain extent, this is already happening. The music social network Last FM currently tracks what its users are playing (see http://www.last.fm/charts for what's currently hot) and Spotify also shows you how popular tracks are as you play them.
In many ways, this emphasis on how many times songs are played is a more accurate reflection of the popularity of music than music sales. After all, a lot of people may buy a track - say, a novelty or charity single like Mr Blobby - only to rarely play it. Equally, a great album, released by an independent artist, may not sell many copies but get repeated plays by a dedicated, small following.
Interestingly, what all this raises is the possibility that some sort of 'song quality' indicator may emerge - something that goes beyond telling us how many people own a song, and lets us know how many people actually like a song. We could arrive at this indicator by dividing the number of plays of a track by the number of people playing it; for example, if 10 unique Spotify users play a song 100 times, you could give the song a popularity rating of 10 (100 divided by 10). But if the same 10 users played another song 1000 times, that track would have a popularity rating of 100 (1000 divided by 10). Crude example, but you get the drift.
Listening to music has traditionally been viewed as an intensely subjective experience, but what examples like the above point to is that maybe beauty does not entirely lie in the eye of the beholder. Perhaps certain songs have intrinsically good qualities that charts have not, hitherto, been able to highlight accurately. Online data could change all that, and start to highlight the fact that some songs have inherent and enduring appeal.
As with so many other things right now, the internet is allowing us to measure our behaviour in entirely new ways - and leading us to make remarkable conclusions about things. I wasn't really expecting the net to tell me how well-written a song is; but it's now looking like a distinct possibility. To conclude, the internet will become a rock critic.