Not Recommended. While the promise of "free" growth is tempting, these services generally violate the Terms of Service of major platforms (Facebook, YouTube, Google). The engagement is low-quality, the risk of account suspension is high, and the "jobs" often pay a pittance relative to the time invested.
Understanding what an "fbsub job" is requires looking at it through two distinct lenses: the engineering infrastructure of large-scale social networks and the world of third-party social media growth tools. Demystifying the "fbsub job" 1. The Engineering Context: Subscription & Batch Systems