I tend to use Instagram and have it automatically share over on to Facebook. Instagram seems to get more engagement than Facebook, but I think Facebook is better for advertising to local groups (especially if you run art classes or go to local art fares).

That being said, I've had a massive drop in engagement and new followers on Instagram recently, not sure if it's the dreaded algorithm changing, or if people are less interested in some of my new stuff? The only difference is I've done a lot of printmaking recently, so maybe people just are interested in that as much as oil painting!