I'm not very good at statistics but if you actually do such a campaign, what do you think of the following additional rules?
- a raffle winner takes it all but then has to leave the campaign forever
- the freed spot by the last raffle winner is of course replenished by only one new participant
- total number of participants of the campaign is constant with a fair and not too high upper limit
Very interesting set of rules. I guess this could give it a taste of: you have to have been part of it.
On the other hand you could calculate how long you will be approximately part of it and i guess most users are not looking for short term campaigns.
Maybe the winner could than also win a spot in one of other campaigns. Or even make the raffle the only way to join another special campaign to increase exclusivity.