The term 'top pillar' refers to the layers of seniority in an organization. Typically the pillars, or tiers, will refer to entry-level positions, mid-level positions, and top-level positions. If hired as a Facebook Recruiter specializing in top pillar recruitment, you will be filling more complex roles such as Product Management, Product Design, User Research, and Analytics.
The interviewer wants evidence that you are ready to handle the responsibility of recruiting for tip pillar roles. Your level of accountability will be high, and your creative sourcing techniques will need to be superior. Talk to the interviewer about the type of positions you have filled in the last 12 months. These details will assure the decision-maker that you are ready for this level of recruitment.