Success is not validation of an idea and we should be ashamed to think so.
Cigarettes are one of the most successful consumer products on earth. Inhaling a lungful of carcinogenic smoke several hundred times a day is undoubtedly a stupid idea. Tobacco has made a small number of people incomprehensibly rich, to the great detriment of humanity.
Personally, I think nearly all of these 'social' startups are bad news. Not as bad news as a lung cancer epidemic, but bad news nonetheless. I think they feed a culture of passivity and attention deficit. I think they fragment human interaction into the smallest possible dopamine-inducing units. I think they're essentially Skinner boxes in disguise - apps that dress up an intermittent schedule of reward as meaningful activity.
The startup culture talks the talk about "changing the world", but in truth most of us couldn't care less so long as we get our next funding round. For every Watsi, we have a hundred bullshit companies with bullshit products, providing yet another means of idle distraction for indolent westerners. We can hardly distinguish between what is worthwhile and what is popular or profitable.