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Meta·Software Engineer·Onsite - System Design / Architecture·Senior

Senior
May 2026

Summary

Meta Research Scientist onsite with an ML system design question on detecting weapon sales. Pretty open-ended, which I wasn't fully prepared for.

Questions Asked (1)

Q1

Design an ML system to detect weapon sales on a social platform.

System DesignTechnical Trade-offsProduct Strategy
Author's notes

This is the kind of question where you think you know where to start and then realize halfway through that you've already gone down a rabbit hole.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and scope, then outline a high-level system architecture that combines rule-based filters, ML classifiers, and human review. Emphasize trade-offs between precision and recall, scalability, and adversarial robustness, and discuss how to measure success and iterate.

Pro tip: Show awareness of adversarial evasion and the need for a multi-layered approach; mention that no single model is sufficient and that continuous monitoring and retraining are essential.

1. Clarify Requirements and Scope

Ask questions to understand what constitutes a 'weapon sale', platform policies, scale, latency needs, and acceptable false positive/negative rates.

2. Data Collection and Labeling

Identify data sources (posts, images, messages, user reports) and discuss labeling strategies, including active learning and handling class imbalance.

3. Modeling Approach

Propose a multi-modal system: text classifiers (e.g., BERT), image detection (e.g., object detection), and graph-based signals; combine with rules and anomaly detection.

4. System Architecture and Scaling

Design a pipeline for real-time and batch processing, including feature extraction, model serving, and integration with human review queues.

5. Evaluation and Iteration

Define metrics (precision, recall, F1, AUC), set up A/B testing, and plan for adversarial adaptation and continuous retraining.

Key Points to Mention

  • Multi-modal detection: text, images, and user behavior
  • Adversarial evasion techniques (e.g., coded language, image manipulation)
  • Precision-recall trade-off and business impact of false positives/negatives
  • Human-in-the-loop review for high-confidence cases and model improvement
  • Scalability and latency considerations for real-time detection
  • Privacy and ethical considerations in monitoring user content

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.