Re: A million bucks won't go as far as you might think...
I am not an Accountant but if this can be classed as winnings on a game then AFAIK it's tax-exempt.
Nor am I a Landshark but if the rules were clear, along the lines of "If you post on this subreddit you agree to be considered as the recipient of the prize or as a donator should you not be picked" then I'd say posting on the SR constitutes joining a game rather than soliciting payment.
The issue there is how this then falls foul (or not) of the US Internet Gambling laws.