Social Network is a really tricky business.
The problem is to get members that can interact which each others in sharing an interest. It sounds obvious but getting them in the first place is really hard and according to your niche (local for instance) it would be even harder.
If you study how facebook took off, it was aimed at Harvard only so easy to get people to join and it spread and expanded from the Harvard starting point.
A capital would surely help, but you need to see a capital as a catalyst rather than a key to the sucess.
An interesting aspect would be to study the social interactions and try to find a way to locate efficiently influent nodes (popular people), and find the best way to get them to join your network.
Also take into account, that it is marketingly impossible to create a need which doesn't exist. You need to target a latent one.
[EDIT] The best advice i could give you is to take some time and perform a reverse ingeneering of your direct competitors' growth
