that's not freemium that's late monetization.
Freemium means you have a product which users get for free but you charge then for parts of the application (premium account at linkedin for example)
When you first do the product and at some point later you do the monetization this is risky. You can do it initially for free but you should have an approach how you want to monetize. As well not to give too much for free initially and ruin your monetization approach.
Facebook is a good example where it worked, Twitter one where it did not so far.
Wikipedia is one where it won't work coz they build up a social oriented community which in the end does not allow for monetisation.
Additionally investors will be difficult to convince when you argue with user growth only, that was different ten years ago.