Based on what I found on web pages, it seems it's not. But since mPFC has intensive neural connectivities with limbic structures, it involved in functions (e.g. cognitive control, memory, emotional regulation etc.) that are usually related to limbic system, and often to be found to have structural/functional abnormalities in psychiatric disorders along with limbic structures. However, generally speaking, it's not a part of limbic system. Can I say that?