Most apps labeled "social" these days should probably be labeled as "consumer", meaning they are intended to do "business" directly with the public.
Consumer websites have one of the lowest barriers to entry in the history of business. You don't have to know anyone, don't have to get a permit, don't have to do anything at all besides make your website and put it online. That's why they are popular to start and stories about them generate so much hype.
Consider this from another perspective for a moment, possibly your friend's perspective. Someone you know made something that you think is really cool and useful, but it probably isn't going to go anywhere. Trying to sell the "B2B" route requires connections, years of industry experience and nice suits. Odds are that neither of you have any of those things (or else I'd assume you'd be using them)
But what you can do is build a consumer app and hope the "social" side gains popularity. The very small chance of it becoming "the next facebook" (or insert ridiculously lofty goal inspired by dhh here) is probably about equal to the odds you'll be able to develop the B2B connections you need to do a good job of selling the product. Business relationships and reputation generally take years to develop and B2B customers are very demanding. It's theoretically possible (but unlikely) that a consumer app will be an "overnight sensation". That almost never, ever happens with B2B apps.
Lastly, we all want useful software. We just tend to talk about consumer software here on hackernews because that's the game we've chosen to play. I'm sure there are far, far less interesting sites out there devoted to B2B software hacking if you look for them.