Peer Review

I've just been updating my CV and, in the process, took a tally of the number of articles I'd reviewed each year. I didn't keep careful records but, by my reckoning, in 2008 (the year I completed my doctorate) I reviewed one article, in 2009 I reviewed four articles, in 2010 I reviewed 8 articles, and last year (2011) I reviewed 15 articles and a book manuscript. So far this year, I've already reviewed one article and committed to two more. Is this going to be an exponentially growing demand upon my time I wonder?