Having worked with personal clouds from their outset (before they were called personal clouds :), I can say with first-hand knowledge that it is virtually impossible to get accurate market data about how many people are really using personal clouds.
This is because the companies that provide personal clouds are either very large or very private and they generally do not want to disclose detailed user stats. If you pay close attention, or you have a wide network of spies or friends in low places :), it is possible to glean periodic glimpses of what various companies are doing, such that it is possible to make pretty good educated guesses about what is really going on in the market.
Today, my company, Funambol, published a report on how 8 of the largest mobile operators in the world have deployed or are launching a personal cloud service based on our company's solution. These operators collectively have more than a billion subscribers. Although a fraction of their subscribers are currently using their personal cloud services, the clear trend is that for operators that have been to market the longest, they are getting significant user uptake. This is because it has taken them awhile to get the word out (and to refine their messaging) and for their customers to understand what a personal cloud service can do for them.
If you would like to learn more, you can download the free report from the Funambol website at http://www.funambol.com.