See, you have it wrong there Ian. You don't ask people to complete a survey. You require them to as part of the conditions of membership of the site. It is pretty simple really.
The site costs a lot of money and time
Most people using it regularly contribute nothing financially towards it (generalising, but probably true)
Advertising is a viable way of keeping the site going
Targeting advertising towards the users is more effective if you have more details about the users themselves
Data collection *might* also be useful for other projects which could be beneficial to recreational flying (depending on going into research design etc)
You can't prevent people from entering bogus information anyway, but you can limit it by not making it too onerous with too many questions, and keeping the details private - declaring that individual information will not be shared, and that aggregate data only will be used to further develop the site. For example, having an extensive background on Bandit12's 350 or so GA hours, including 1 written off Tomahawk and 1 airspace incursion in the last 15 years is of little use by itself. Knowing that ~150 predominantly GA pilots access the site at least weekly may be of interest to advertisers, and also may help in allocating development resources, especially if it turns out that ~500 predominantly RA-Aus pilots also access the site weekly (numbers made up of course).
As David Isaac says, make everyone go back to add details to their profile. Maybe yearly force it again to check/update information, and make the forums not visible until the profile/survey is complete.