A common mistake of a lot businesses make is in treating each network the same, and assuming that the users also behave similarly.
People usually actively engage with one or two social networks usually multiple times daily, and use others less frequently. They pick their "favourite' based on how comfortable they feel within the environment (the user experience), connections they have (friends or contacts), and the purpose for which they are using the network (socially or professionally).
When it comes to how brands use social media however, there tends to be a hierarchy of importance based on size and potential reach --- particularly in B2C. Facebook tends to take the lion's share of interest, which is hardly surprising given that it has 1.71 billion users, but for some, it may not be their best opportunity.
Quite often though, content is planning for Facebook and just 'shared out' to the other networks without much consideration of whether it is engaging to those audiences, or if they are even there. The same could be said fro B2B messages on LinkedIn that are broadcasted to more consumer-centric networks.
For example, you may have an actively engaged brand page in the UK, but in another country, perhaps Twitter has a better community to interact with. Age is also a factor with which identifying which could be the most effective for your brand.