The way I see it social sites need to climb a very steep peak before they have any chance of making money from ads - how many facebooks (which is still a long way from profitable) can the web ecosystem support? - If you do reach the top of that peak which only has room for 2-3 big players you have a chance of banking millions.
B2B services on the other hand are much safer. Business people understand that products and services cost money - and they are used to paying for the services they use.
My advice to developers looking for a product is to forget about the 1 in a million chance that they will develop the next great social app and cash out rich. By focusing on customers that are willing to pay, your venture has a much greater chance of becoming profitable and sustaining itself.