Just my observation - seems like there is less listed on AG

I think one of the issues is definitely that audiogon doesn’t condone conversations between members outside of audiogon

U S Audio mart has certainly progressed in the last few years and the buyers as well as the sellers have all upped their game, with that said I still use Audiogon to buy and sell when needed as what they offer is unique and on a larger scale, plus the forums are great to read and reference when needed. I do pay the $50.00 membership to USAM to give back for the free ads, which I have used quite a bit lately. Enjoy the music

USAM  Much easier to use. Communications between seller and purchaser is great, and it's free. Post all the pictures you want. It's great for our hobby!  Can't beat that.

Could Craigslist be a major reason you see fewer online AG listings?  

Doubt it

CL has been there since the beginning