To join or not to join. When it comes to new social sites, that is the question brands must ask themselves.

While social networks like Facebook and Twitter continue to be dominant, services like Pinterest and Instagram are attracting more and more individuals. Even Google’s social network, Google+, which many were skeptical about, has managed to grow into a respectable channel with more than 100m active monthly users.

For most brands, the growth of social as a channel is a great thing, but the conundrum — which ones should we join, and which can we ignore — is increasingly complicated.

Here are five areas brands should look at when answering that question.

1. Functionality

Thanks to creative entrepreneurs and innovative companies, brands have an ever-growing number of ways to interact with consumers online. In some cases, it may be beneficial for a brand to experiment with a new social networking service, however nascent and unproven it may be, because it offers unique functionality that other services don’t. Even if the new network doesn’t take off, the discoveries could become useful and relevant as existing social sites evolve and add new functionality of their own.

2. Demographics

Not all social sites are created equal when it comes to demographics and brands considering setting up shop on a new network should take this into consideration.

According to a new Pew study, Instagram, for instance, reaches a higher number of high-earners. Despite the fact that Instagram, while growing rapidly, isn’t the largest social network in the world, its demographic makeup makes it an attractive place for luxury brands, and it’s not entirely surprising that many of the companies putting the mobile photo-based social net to the greatest use and attracting the most followers are indeed luxury brands.

3. ROI

Social may be a challenging channel for brands to determine ROI in, but ROI isn’t a completely unknown quantity.

For retailers considering their social investments, for instance, there’s a decent amount of evidence that clicks from Pinterest are far more productive than clicks from Facebook and Twitter. Does this mean that retailers should invest more overall in Pinterest presences than their Facebook and Twitter presences? Of course not. But for a retailer looking at a Pinterest investment, data with ROI implications shouldn’t be ignored.

4. Overlap

Many consumers are active on multiple social networks. For brands, this can be both a blessing and a curse. On one hand, it gives brands opportunities to create stronger relationships by putting multiple channels to work. On the other hand, it can lead to duplicative effort and investment that doesn’t deliver results that scale as desired.

When evaluating a new social site, brands should analyze just how much overlap there is between the social channels they’re already present in and the new site. While some overlap is to be expected, higher levels of overlap may suggest that resource instead be committed to new channels that can bring new functionality, audiences and engagement types.

5. Risk

Brands face numerous risks when joining new social sites. At one extreme, they risk investing in services that won’t go anywhere. On the other, they risk not getting in on the next big thing early when competition and costs of user acquisition are lowest.

The costs of either can be huge. Staff time is not cheap and opportunity costs can be significant, but missing out on the opportunity to jump on a trend early can be expensive. As a result, brands should assess the risks they may be taking when looking at new social networking services.

How to quantify risk? Plotting functionality, demographics, ROI and levels of overlap against existing social investments is a good start.