Here's how I'll select the winners in a verifiable way:
I've created lists as replies to this post, with a number for each entrant. They are verifiable on the blockchain.
For each contest (retweet, upvote, reblog) I will choose a coming Bitcoin block which has not occurred at the time of this post.
Retweet: 422935
Upvote: 422936
Reblog: 422937
For each contest I will take the hash of the block, and get the mod of the result vs. the number of entrants. Mod(hash, total_entrants) + 1 = the winner.