Funding is only really needed if you really need the funding. It's really a case by case basis. I've started some things with only the money in my pocket. Other things required me to find angel type funding among my network of business associates. I always try to make it as far as possible via my own funds and effort, as that has always served me best.
The success or failure of a social web app is impossible to predict. I'd say it's similar to predicting the stock market. You should look at the fundamentals of your plan and see whether your business idea really has the possiblity to make money. Make sure you understand marketing and sales and evaluate whether the product is marketable.