Cameron Moll:

While we don’t pay services such as Tumblr and Twitter in monetary means, we do pay them in usage, not to mention attention given to any revenue-generating services (i.e. ads or sponsored items). This usage and attention is what attracts the huge sums of investment dollars these kinds of companies acquire. In the end, if our attention and usage cannot be “paid” to these free services, it’s likely we’ll pay them elsewhere.

A great way to look at the relationship Tumblr has with its users.