I know that Horizon airlines, owned by Alaska still does but nowadays it’s difficult to find out the routes that still use Turboprops. And I heard these are being phased out quickly.
The short answer is: a lot, but dropping fast.
Working in the aviation industry I have access to some fun tools so I thought I'd look this up for you. Today (29 November 2017) there are 2,174 scheduled turboprop flights in the US.
Many of these (874) are in Alaska, but some other notable routes, in addition to the Horizon routes you mentioned, are run by Silver Airways, who fly Saab turboprops all over Florida and the Bahamas. American Airlines still has some old turboprops flying out of CLT and PHL, though those will be retired very soon (these are flown by regional airlines, not AA itself). United still has some regional flying on turboprops out of IAD as well.
Some other smaller turboprop operators with fairly large operations include Cape Air, Great Lakes Aviation, and Southern Airways Express.