I would suspect that it would be related to Android development given that (1) Android SDK requires Java, (2) Facebook develops a native Android app, (3) Facebook does NOT develop a native Blackberry app (other mobile SDK using Java), and (4) the identified engineer(s) were on the mobile team.
Having the Java plugin installed does not mean they were doing Java development. It could have been iOS engineers and they had the plugin enabled so they could access some internal application.
