I think its unrealistic to compare any social networking site to Facebook. FB served a need that nobody realized was there. Mark Zuckerberg capitalized on it. He's said in an interview once that 8-10 weeks into FB he seriously considered moving to the next project.
This wasn't his first project, rather the first that really took off. The key is to keep at it. When it comes to social networks, if you build it - nobody cares. A couple years ago a few friends and I undertook the same challenge, to create a social network that kept people returning. To say it was difficult is an understatement. Marketing and driving traffic gets exponentially harder the fewer funds you have allocated towards it. If you're bootstrapping, be prepared for many setbacks.
I'm not saying it isn't possible, just difficult. It only stands to reason that FB will fall one day and another shall take its place. Where you may have a much higher degree of success is with niche markets. If your social network is catered to a niche market, interest level and participation drastically increases.
I think your first step is to map everything out. If you're serious about this, you may consider:
1) Do the market research - You need to know what you're getting into. If you don't know your market, any direction you take will be a roll of the dice.
2) Create a business plan - Some may disagree with me here since a business plan is sometimes seen as a formality. However, if done correctly, your business plan is your map and will always keep you on the right track. Your business plan should be revisited often and revised accordingly.
3) Create a marketing plan - Marketing will be your X factor. It will either make or break your social networking site.
Good luck with your site!