Exactly. Yahoo! is buying popularity not market position.
The meaningful stats that are not referenced is the change in churn rate over time (becoming popular to people that don't know you is only good if you are remaining popular with the people that do know you).
the odds of failure of a new business don't necessarily change just because it is social or digital. often these traits mask the fact that there is no real need being served, or no actual, sustainable business model. the social/digital attributes often lead people to conclude that true need or business model are unimportant and can be developed later (or will magically emerge). the focus on experience also misdirects, as in: if we simply do it better or in a more engaging way, customers will come...this is not meant to suggest that the category doesn't have potential or that there isn't opportunity to consider. but at the end of the day, if you aren't bringing in more money (either revenue or investment) than you are paying out, a business is not sustainable.
"Maybe I’ve gotten busier since then. Or maybe in practice, the rewards just aren’t exciting enough."This is the key point. If there isn't enough value for the customer/user, the novelty will eventually wear off and use will drop off as well. The challenge is to sustain a monetizable value proposition (which can actually be very difficult to do), not just have a fun app.