Quotes from a recent study by ZeroTurnaround:
"Java 7 has been out for less than a year, and at 23% adoption we were slightly surprised at the speed of uptake."
"Java 6 is the overwhelming version of Java, used by 88% of respondents. But more interesting and exciting is the fact that 23% of respondents are already using Java SE 7. This is amazing penetration, considering it came out less than half a year before the time of this survey. This gives hope that as Java SE 8, 9 and 10 come out in the next 6 years or so, a lot of engineers will be able to benefit from the changes quickly."
Refer to the study for more detail (registration required, quotes used with permission). This data matches other indicators, such as the 18% adoption rate posted by Jelastic a few months ago, which goes on to say:
"This growth is especially impressive considering that Java 6 is currently the default option in Jelastic, so users who turn to Java 7 make this selection explicitly."