They can use BitUSD
Create ReddUSD & back it with BitUSD. (This allows micro-payments & transactions within Reddit) Within the system you can earn and tip tiny amounts like $0.01 ReddUSD but you need >$1 ReddUSD before Reddit will cash out a BitUSD.
How - Reddit can allocate 20% of their advertising spaces to ReddUSD, perhaps a specific front page spot that is auctioned off once a week, thereby making Redditers shareholders and immediate beneficiaries of revenue.
If they made $100 000 in advertising revenue in week 1 from that spot, it could be divided among the Reddit users based on the Reddit points (using their current system) they earned that week.
(I.e If 145 million points were awarded in total to Redditers that week and I got 700 points myself, then I would make $0.5 ReddUSD which I can use in the system and they can track for free in their centralised software, but once I accumulate $1 of ReddUSD then I can withdraw 1 BitUSD. It should work & be transparent provided the advertising auction is done in BitUSD and paid into an account people can check, though of course if Reddit goes bust, your ReddUSD will be lost.)
Other:
If their current system doesn't already do it, perhaps there should be more points awarded for creating popular reddits and also for anticipating the consensus. (Up voting a good reddit before it became popular and vice versa.)
That advertising spot would be extremely popular and the auction for it would also generate a lot of interest too, so they should really be able to get a lot of value out of 1 good advertising space that they give to their users. (So even if they just gave users 50% of the revenue from that one spot, the increased exposure might mean they earn double from it than they did before so Reddit might not even lose out at all overall.)
I see they already have 'Reddit Gold', BitGold would work above too.