Charging is not a bad solution.
Give people time or features for free so they can try your product.
If they like it, they will gladly pay for it.
To avoid losing valuable users you can:
Charge less for people that are contributing to the platform.
Charge less from ‘small fishes’. Let the big companies that you mention pay a little more.
When you monetize your product, you make your platform less organic.
Promoted posts on Twitter and Facebook kill diversity. Being flooded by ads kills diversity. Having a few ‘influencers’ that invest money and make Twitter and Facebook a business kills diversity.