SQL (easy to medium, no need to practice if you use it weekly basis). I would focus on different products like instagram, groups, marketplace. Try to write down few metrics (not just the concept, be specific) for each of them. No matter how much you practice your answer would lose structure, because the interviewer would interrupt and take the answer into the direction he wants. Best way I can think of is to understand all products and kind of build FAQs in your mind about the product. Focus on A/B testing fundamentals and also drawbacks(network effect, unit of analysis...), challenges, ways to avoid them.