I can foresee an awesomely blackhat use for exploiting this. Remember back in 2008-2009 when there was a major attack that relied on SEO manipulation to direct susceptible users to a page to download "video codecs" (actually a slurry of malware)? Yeah, I can bet that Google is going to find themselves in a race to defeat similar manipulation. How many people have a friend/coworker/acquaintance who friends anyone and everyone on Facebook or any other social network? All it takes is for one of those to be a front for a botnet owner. The botnet +1's the link to a redirect, it gets recommended to everyone connected to the front account, some of those individuals follow the link and get infected, and then their own newly botted computers start the process all over again. Hooray social media worms!
Now, I know from my own research that Google does do some detection and flags potentially malicious sites. But if this attack were coupled with a 0day or an undisclosed security hole, it could have decidedly non-trivial impact.