Because overinvestment in a specific sector of the economy necessarily results in underinvestment in other sectors of the economy.
In the case where investment capital flows to ultimately unproductive enterprises like photo sharing, and away from productive enterprises like efficient batteries, cancer drugs, etc, humanity suffers in the long term.
Now it's possible that the proliferation of photo sharing services is increasing the efficiency of photo sharing that people were doing previously, thus freeing up time to engage in other productive pursuits, so it's not necessarily a bad thing to sink investment capital into ventures that appear unproductive in and of themselves.
But I would highly suspect that this is not the case for the current bubble. It's quite likely that photo sharing apps are actually generating demand for photo sharing activity, thus being a net drain on actual economically productive activity.
In the case where investment capital flows to ultimately unproductive enterprises like photo sharing, and away from productive enterprises like efficient batteries, cancer drugs, etc, humanity suffers in the long term.
Now it's possible that the proliferation of photo sharing services is increasing the efficiency of photo sharing that people were doing previously, thus freeing up time to engage in other productive pursuits, so it's not necessarily a bad thing to sink investment capital into ventures that appear unproductive in and of themselves.
But I would highly suspect that this is not the case for the current bubble. It's quite likely that photo sharing apps are actually generating demand for photo sharing activity, thus being a net drain on actual economically productive activity.