This scheme seems to be getting more and more popular. What value to you think having a big giveway shared by 25 or more bloggers has?. They all blog about the giveaway and the all link to the 25 blogs or more sharing the giveaway. Its creates one big link farm. Can there be a downside here, for example google viewing this as trying to game them?
There's 25 bloggers and only three positions in the top three of the SERPs, and ten spots in the top ten of the SERPs. I don't think all twenty five blogger are going to benefit. It's only twenty five links. If the SERPs for that niche are that easy to game then the barrier to entry is set low.
I see this a lot, but I don't know how successful it is. What you're likely to get is a big footprint of similar text, and a linking ring. That's not natural, but there are already supposed to be ways the search engines can detect and penalise that behaviour. If they're not catching it we can look forward to a statement from Matt Cutts telling bloggers to stop doing it, and while they're at it to stop selling links and making spammy guest posts.