10k is the short answer. The # demonstrates interest beyond your personal networks. Engagement metrics of those 10k is important - how long users spend on your app/website and how often they return. The longer the engagement when on the app the better for social. But for frequency I'd show the number of users that come back >= 1 x month for 3 months. If you have higher frequency, show that instead of monthly actives.
They will also want to know your speed of growth, how you'll become profitable and how long it'll take. They want to make money, tell them how. If you want to sell advertising then you will have to have higher engagement and frequency. If you want to sell prints of those nostalgic pictures to the members and you're making sales, those frequency numbers can be lower.