65.2039489134935842093585%
LOL!
As stated, it is impossible to know what proportion. It may only be about 10% these days and dwindling due to the shifting focus from development and technical/cryptographic issues. I for one used BTC for over half a year before even knowing of the existence of this forum. It all depends on what it is used for. For instance, I used P2P lenders and thus would have no idea about this forum, except some veiled reference to a "BTCTalk account".
I would say though that the marketplace subforums are growing in popularity and bring Bitcoin users to the forum; if the particular services they use do not function as expected, here is often a good place to contact their support teams.
Also bear in mind that many people use Bitcoin from international countries, and though while here does have language boards, it is a predominantly English-language forum. BTC may also be held by citizens in Venezuela, Greece or Cyprus (i.e. after their sovereign debt crises), and they are unlikely to come here, even if they do have regular internet access.
Very grey area; this is only a cross-section of bitcoin's activity.
