Did the people who had SERP drops after adding the button utilize it in a BH fashion? If you pounded it with +1s (and social media relationships on a fast paced mass scale are not usual for your site) then that +1 could have been THE INDIRECT CAUSE of your SERP drops.
With the information given, the first thought is that you have your tin hat on and looking for the FBI bugs in your basement.
The reason I entertain donning the metallic topper in theory at least is that if +1 does become as popular as liking, friending, or digging etc.. then the direct impact of social media on SERPs will be a lot more "analytics" (capitalize that if you wish to, the pun is intended) and less third party backlinking.
I can easily foresee a future (or a return to circa 2002* in many ways) where if you are not using Google, you are not in Google. I think that the +1 button was the first exploratory foray in this direction. I do not think that this is the end game piece of the puzzle, but they seem to be trying to reverse a decades trend that they started.
2002:
Code:
Yahoo! was founded in 1994 by David Filo and Jerry Yang as a directory of websites. For many years they outsourced their search service to other providers, considering it secondary to their directory and other content features, but by the end of 2002 they realized the importance and value of search and started aggressively acquiring search companies.
Overture purchased AllTheWeb and AltaVista in 2003. Yahoo! purchased Inktomi in December, 2002, and then consumed Overture in July, 2003, and combined the technologies from the various search companies they bought to make a new search engine. Yahoo! dumped Google in favor of their own in house technology on February 17, 2004." -http://www.searchenginehistory.com/#yahoo
While this might seem unrelated in the extreme to many, with an aluminum helmet I see a direct connection. When Yahoo got Inktomi, they also were able to add enough infrastructure, personnel, and especially cash, to keep up with the needs of autocrawling for the cache, sort, redisplay and dump (Google has always come under fire for their no-dump policy).
They were forced to do this to gain market share in an ever increasing demand for search technology, that was following the math of Moore's law.
While there were limited resources, and even fewer dollars, the marketing model in many minds was one of subscription. LexisNexis was in the head of the pack, as well as private collections, LAN style "nodes" with search capability. Yahoo had gained a widespread popularity for its free directory of online resources, which was soon being eclipsed by a fast moving, freely open search engine called AllTheWeb. People would come to it and add their webpage to it like a directory, but was able to handle large numbers of links and information with greater usage convenience.
With the Internet on its fast demise into the "common mans" hands, the subscription model was not going to be the winning model, period, and with all of that Silicon Valley money, it would be a lot easier to upscale your equipment and tap into advertising, instead of simply catering to an upscale market of pioneering techies or money laden "gotta have its". AOL failed at this and died out as a power player.
How does this relate to Google's +1?
I think that Google might be looking for a way to put the genie back in the bottle.
With the name of Yahoo sounding a lot like dirt now a days, especially with all of its developer tools disappearing (AKA site explorer, overture, etc), Google finds itself in the quite comfy position of being at 2002 all over again, but with FB,MS, twitter and ilk as the #1 threat. Well guess what, everyone who spends money on marketing only uses social media to get on Google (I know this is a slight exaggeration).
Just like the man pouring beer into the toilet at the party. "Why make yourself dizzy and sick being the middle man"
