Yes. I'd stake that the majority of retweets today are in fact bots. See below...
"OK. 1 more chance. If no one RTs, replies, or even talks about this tweet, I will donate £15,000 to a charity of your choice. Any questions?" --> 369 retweets, most by suspected bots.
I'm confused. Did you mean contests (as written) or contestants? I agree that bot vs. bot is nothing to lose sleep over, but I thought we were talking about bots vs. actual human fans who want to win an autographed knick-knack or a pair of tickets to an event or whatever.
I'm not big on competitions or surveys, but I occasionally participate in one spontaneously for amusement, and on those occasions where I've won something it's been highly enjoyable. I'm much less inclined to do if I think about mechanized competitor-bots or suchlike, and think that automating such activities is astonishingly selfish.
"OK. 1 more chance. If no one RTs, replies, or even talks about this tweet, I will donate £15,000 to a charity of your choice. Any questions?" --> 369 retweets, most by suspected bots.
https://twitter.com/rickygervais/status/324632844829876224