Looking at the traffic statistics to this site over the last month, nearly 40% of referring keywords are listed as 'not provided'. The 'not provided' status means that the site visitor is using the secure https version of google. This is the default for people who are logged in to google services.
Ironically, the more people that use Google + the less useful google analytics is. Until google do something to remedy the situation (and i know that they could if they wanted to) confidence in the keyword referrals statistics is going to erode further.