The "Like" button on Facebook (and now all over the web) allows a user to indicate positivity about a piece of content without actually writing a comment. Millions of people have "Liked" status updates, notes, photos, bios, comments, and now external web pages. Similar one-click sentiment links ("Endorse," "Helpful," "Not Helpful") now exist on LinkedIn, Quora, and other social networking sites. And of course "Upvote" and "Downvote" arrows have driven sites like Reddit for a long time.
Take a blog post. Historically, the only option for a user to engage was to leave a written comment. Suppose I write a post and five people leave comments. With the addition of a "Like" button I would estimate three people would leave comments, but four people would click "Like." Before sentiment buttons: 5 written comments. After sentiment buttons: 7 engaged users, 3 commenters.
My friend Dario Abramskiehn asked me awhile ago why certain of my posts receive more comments than others. I follow Chris Yeh's theory on comments: the less serious / difficult / lengthy the blog post, the more comments it will have, assuming an average level of interestingness. I chalk this up to the law of reciprocity: if you take the time to crank out something really thoughtful and original, readers feel like they reciprocate the effort in a comment. So many, naturally, abstain. By contrast, if you post something provocative and short, it's easy to leave a quick comment and feel square with the effort of the blogger.
With a "Like" button, readers who would have previously abstained can now indicate passive positive sentiment. Some readers who previously left a comment but did so half-assedly would now click "Like."
The generic, safe nature of "Like" also increases total engagement when difficult topics would otherwise deter readers from chiming in. A friend recently posted a status update on Facebook about a relative's fight against cancer. It was a positive update — i.e., one that warranted congratulations or encouragement — but, given the sensitivity of illness topics, given the fear of offending someone — the status received quite a few "Likes" and almost no comments. By contrast, a message that's more straightforward — such as my tweet about how many libertarians are religious — received several comments but no "Likes."
Bottom Line: The "Like" feature and other passive sentiment links next to content on the web show that the way users engage with content will continue to change, and that the way to measure the vitality of an online community continues to be more complicated than raw numbers such as unique visitors or numbers of comments.
(thanks to Steve Dodson for helping brainstorm this. P.S. "Like" buttons coming to this blog soon.)