There isn't one single "best" platform because it depends on who your investors are and how they prefer to find information. That said, here is how the main ones tend to work for off-market deal sourcing (meaning properties not listed on the public MLS, which stands for Multiple Listing Service).
Facebook still has the largest pool of local real estate activity. Groups built around wholesaling, fix-and-flip, or local investing communities are active and free to join. Direct outreach to owners through Facebook Marketplace also works for some deal finders.
Instagram skews younger and works better for brand building than for deal sourcing directly. If you post consistently about the kinds of deals you look for, you attract inbound leads over time, but it is slower to produce results than Facebook.
LinkedIn is worth considering if your investors are commercial buyers or high-net-worth individuals. The audience is smaller but tends to have larger budgets.
YouTube is a long game. A channel showing deal analysis or market tours builds authority and pulls in both sellers and investors, but it takes months before you see meaningful traffic.
One thing worth knowing: the platform matters less than the clarity of your message. A post that says exactly what you buy (location, price range, property type) converts far better than a generic "looking for deals" post on any platform.
The strategy guide on the Guide tab covers how content quality separates durable providers from ones who get lost in the noise, which is relevant whether you are finding deals for yourself or building a service around it.
What types of properties and what price range are you focusing on? That would help narrow down where your specific audience actually spends time.