I think Production Engineering is especially interesting to me as I think we do a very good job of lowering the wall between ops and dev, mostly we build tools and do evangelism to help SWEs own their own services, but above all we do whatever is necessary to keep the site up in the most scalable way we possibly can. This includes engaging with our product engineers, not just infrastructure, and helping them understand the impact of product on infra and vice versa. That's one thing I especially like about PE, there is very little of the ops/dev divide, it's more of a partnership with PE and SWE both helping to own the service in production. (disclosure, I work at FB as a PE)