Your argument would carry more weight if your analogy were better. But the fact is that social networks are what Dijkstra would have called a "radical novelty":
"The usual way in which we plan today for tomorrow is in yesterday's vocabulary. We do so, because we try to get away with the concepts we are familiar with and that have acquired their meanings in our past experience. Of course, the words and the concepts don't quite fit because our future differs from our past, but then we stretch them a little bit.... It is the most common way of trying to cope with novelty: by means of metaphors and analogies we try to link the new to the old, the novel to the familiar. Under sufficiently slow and gradual change, it works reasonably well; in the case of a sharp discontinuity, however, the method breaks down: though we may glorify it with the name 'common sense', our past experience is no longer relevant, the analogies become too shallow, and the metaphors become more misleading than illuminating. This is the situation that is characteristic for the 'radical' novelty." (EWD 1036)
Hammers don't care if we use them or don't. There isn't one big Hammer.com that produces all of them. Hammers are not constantly acquiring new features. Hammers aren't useful or not on the basis of how many other people use them. Hammers do not even have significant overlap with other tools. Hammer.com doesn't need to convince anyone to try hammers. Companies that make hammers want you to buy their brand, but they don't bother with trying to convince people they need them.
Look at all the differences from social networks. Unlike a hammer, if nobody else is using LinkedIn, it's not useful to you anymore either. If everybody quits using LinkedIn but you, LinkedIn won't have any revenue, even though you're not paying to use the service. If everybody quit using hammers for stupid reasons, there wouldn't be anything stopping you from using them; in fact, any hammers you already own would continue to work. LinkedIn could just vanish despite your protests.
LinkedIn is pretty unlike a hammer, so I don't find your argument holds all that much water. It's a social network, so it's of interest and on-topic here when people decide to leave. It's interesting, because many people here are building their own startups and many of them are social networks. They survive or die on being used. If there were great insights in this article about how to retain users, it would be especially interesting, but knowing that there is a general malaise about this one is still information, and the article is still well-written and I still feel I got my nickel's worth from it.