You can see some decrease in the number of contributors:
https://www.ohloh.net/p/gnome/contributors/summaryHowever, note that the current slide in the numbers is something that only happened after February-March 2012, which was when the previous major release was launched. In 2010 another slide lasted from April until October, so I suppose there is a need to worry if contributions don't pick up around Sep-Oct.
That said, the situation for GNOME is certainly different now, given that they lost their biggest distribution channel in Ubuntu doing their own thing.