Can Google be used to measure the popularity of something? Maybe. That is the approach of the famous TIOBE Programming Community Index. ( See http://www.tiobe.com/tpci.htm).
As the maintainer of an open source project, it is something I look at. I am happy to that ehcache now returns more results than any other Java cache with 348,000 results.
So, is is the most popular? Who knows. But downloads and google suggest it is.