As the guy who wrote one of the first open-source status update systems in 2007, the main developer behind Acquia Commons social business software 2.x, and an evangelist of social communications technologies, writing the title of this post feels strange. I've spent the last 5 years of my life building software to make it easy for people to build social networks, so why would I suggest that sometimes you shouldn't do it?
Mainly because the term has become so overplayed that too many people think they can just "make something social" and it will magically make users want to come sign up for their product.
In my conversations with entrepreneurs, many of them say they want to build a social network. Then they'll say "it will be a website where people/businesses will do X." Look -- if you have expertise in X, build a company that specializes in X. Social networks first and foremost solve communication problems for their users. A website that is primarily designed for businesses to get feedback or for people to buy event tickets is not a social network (although it may benefit from incorporating social technologies). On the other hand a website that is designed for people to talk about businesses or about events may be a social network that can monetize by taking advantage of interest in those topics. When founding a startup, though, you should focus on what you know you're good at; startups are hard enough without having to learn a whole new field or find another co-founder with the right experience. So instead of building your social website for businesses to get feedback, consider building a widget that businesses can incorporate into their own websites.
Social communication technologies are a crucial way to incorporate network effects into a business. However, some businesses would be better off focusing on a different network. Ideally the people who pay you are the same people who use your product. You can't "make something social" as a drop-in solution for finding eyeballs or the consumer side of your market. If that's what you're looking for, I recommend leveraging systems that already have giant scale.
If the problem you want to solve really is about communication, then by all means build a social network. I've built the tools you need to make it easy to do so. If you're not a startup and you want to find a new customer base or build existing customer engagement, it's absolutely worth considering social technologies. Helping people to express themselves is a worthy goal. And if you're in an industry that needs a shake-up, social technologies could inject new life by promoting customers' voices. Just remember that social network users are customers too, and it's just as hard to build that audience and keep them interested as it is to build any other audience. If your social network audience is not the same as your customers, then unless you're a source of entertainment selling eyeballs/data, maybe what you're really looking for is crowdsourcing.