If you look back you can see we've been growing at a pretty steady clip every since we launched: http://trends.builtwith.com/analytics/Optimizely
However you are right that there does seem to be an inflection point in May 2012. I can only speculate why this happened. Here are a few possible reasons:
1. We got really lucky with a bunch of great press at the end of April including a phenomenal piece in Wired: http://www.wired.com/business/2012/04/ff_abtesting/ and a piece on CNN: http://www.youtube.com/watch?v=TEpsfQHNUxU
2. We announced our funding: http://blog.optimizely.com/2012/05/30/optimizely-surpasses-g...
3. Probably in some part because of #1 we started ranking very high for organic search terms. For example we are now the first ranked result for "website a/b testing" on Google.
Hope that answers your question!