It depends on the type of users (if this is a network) and the type of supply (if this is a marketplace) + any geographical constraints. If you have geographic constraints (users need to be close to each other to interact), it's most effective to start in a very small local area and build critical mass there before expanding to neighbouring areas. In case of a network, tap into existing communities that are already interacting and attempt to bring those interactions onto your network.

if you are planning on launching a social network .. the most common strategy to follow is Piggyback on top of a thriving network, for example Paypal got almost all its traction by piggybacking on eBay and offering a much superior payment method than the painful check-over-mail. Here you can find more info

https://medium.com/@sanguit/stealing-traction-how-youtube-paypal-stumbleupon-and-airbnb-grew-through-piggybacking-fbced00417e6