Web users in the US, UK, Brazil and Japan are spending even more time on social networking sites like Facebook and Twitter, according to The Nielsen Company.

The research firm has reported that the number of people using these services is increasing, and that this audience is spending more time sending messages, uploading photos, and so on.

Overall, 140 million Americans spent an average of over six hours on sites like Twitter and Facebook in December 2009, Nielsen said.

The growth in popularity of social networking in each of the ten markets assessed by the company shows just how important it is for marketers to “join the conversation”.

What is less clear-cut is how brands can reach this target audience, and establish the return on investment from this activity.

Perhaps Forrester’s latest “social technographics ladder” will be a good place to start.