That's the year Google killed Reader, and all the mailing lists I'd been on since the '90s spontaneously moved to Facebook. I think it was also the year mobile really took off
The decline was definitely underway. 2010 is also the year Duke dropped usenet. But the blogosphere was still going strong and mailing list traffic was still strong. Facebook group versions of many mailing lists and private forums existed but discussions were high-quality and didn't cannibalize the parent forums much. Forum/list traffic collapsed in 2013 and meaningful Facebook group discussions followed about a year later.
http://tumblr.jackdawresearch.com/post/136750883723/digestin...
(The last two charts are trailing-year, which better show the derivative than the more typical cumulative charts)