When users write a comment (such as what I'm writing right now on Hacker News), they are acting as writers who generate text for people to read. When they submit a link, they are locating content for people to read. When they upvote or downvote, they are acting somewhat like editors by helping to curate content.
In traditional media (a newspaper, for example), these functions would typically be performed by paid staff. In social media, it doesn't work that way.
But then the whole point of social media is to interact with other people. And interaction is two-way. You could argue that the value users receive from interacting is the reason they come to the site. And consuming content is part of that value, but you could also argue that being heard is part of the value you receive as a user.
When I do any of any of these things, am I doing labor by giving up my valuable time in service of the business? Or am I receiving value because the site allows me to be heard? Maybe the answer is both. Probably we should look at every aspect of the transaction.
Continuing the newspaper comparison, look at letters to the editor. Does anyone argue that people who write letters to the editor are unpaid volunteers? Not that I know of, because everyone understands that people write those letters because they want their opinion to be heard. So even in traditional media there's a little precedent for this.