All the jobs which recruiters send me all day long seems to tell me to look into ruby. Or do loads of people start projects in ruby, then leave for something else?

I don't see quite as many ruby jobs today but they're still out there.

Ruby has been very focused on increasing performance the past few years. A few major teams ran away from Rails because it's really easy to build functional things in Rails that won't scale, which gets attributed back to Ruby performance. I think that was enough to scare people away from using ruby at all

GitHub and Shopify both use Rails, among many others.