Fb e5 mobile focus interview

Im about to have an onsite at fb for a senior swe role with a focus on mobile. Recruiter mentioned the system design round but didn't say much about it. For folks who were in a similar shoe, what was your system design interview experience like? Was it something like "how do you architect the fb app for mobile", or was it more like "design fb", or "design tiny url" type of things? Thanks in advance