In the dotcom bubble of 2001 I heard the same thing except with "internet" replacing "social media".
In the last bubble there were no customers. Facebook makes $4 per user. The users are customers. They produce real revenue. Nobody's debating whether Facebook can make money.
This may be true but how much of that $4 per user is driven by the bubble itself? How much advertising is coming from companies that are within the Silicon Valley gravity well? If the (so-called) tech bubble pops, people will still need cars, vacuum cleaners and food so how well does Facebook do with those kinds of advertisers?
The reason a bubble is scary is because it distorts your normal metrics of what a healthy company is. It doesn't matter how much money you're making selling hotdogs at the boardwalk if one day people stop going to the beach.