Ask HN: What percentage of pre-launch/beta users register on full launch?

I think it can be safely assumed that not all people who pre-register would respond when a full launch is announced.

Can those with relevant experience share the percentages?

Also, I think it would be interesting to discuss the possible causes of drop-outs and various factors affecting this. e.g. the longer the period between pre-reg and launch, the higher the percentage of drop-outs; the more useful/valuable the service, the lower the percentage of drop-outs etc.

Please share what you can.