I would first point out that it is not really a claim but a simply observable fact for anyone with a brokerage account, check the available market depth at any given price to see how many shares can be sold into the available bid before having to sell at the next lowest available bid in the auction.
The parent did not make a claim that comparing the market cap of TWTR to that of FB is useful, nor am I attempting to make an "argument" in the opposite direction. The goal was to point out that measuring success by an extremely volatile marginal price which rates the stock at sixty-six times earnings is neither accurate nor useful. A little surprised that my attempt to provide useful information earned a handful of downvotes compared to the parents throwaway sentence.
If one honestly believes that market cap data is useful for comparing publicly traded companies in anything but the broadest sense, then one must believe that yesterday TWTR was "worth" 2.5% more of FB than today (judging by daily fluctuations in the TWTR:FB ratio). That is observably not the case, the only difference between that day and the previous is that the company made some information public, no facts at the coalface actually changed. Value of equity is decided over the very long term life of that equity.
As for your final question, one need only look back in history to see that the aggregate marginal daily price and hence market capitalisation is often (and especially in tech) not a good estimate of future expected earnings of a company. The market almost always overshoots, both to the upside and downside, largely depending on available liquidity.