They're not just closing ShareCG, though. They're closing a bunch of their web sites, some of which don't offer free stuff.

"After many years of serving the engineering and design communities through our network of websites — including EDACafe.com, GISCafe.com, MCADCafe.com, AECCafe.com, TechJobsCafe.com, and ShareCG.com — it is with deep regret that we announce the upcoming closure of all platforms due to financial challenges."

I think this is a result of the crash in ad revenue. Traditional web sites are hurting, as people migrate to social media. They're going the way of a lot of other sites. Buzzfeed News, Vice.com, Jezebel. SB Nation cut coverage of many major sports teams. Daily Kos has admitted they're struggling and have had to layoff staff. 

It's becoming very difficult to support a web site on ad revenue.