I'm sure it isn't easy for Twitter to make money, but their reputation is still in tatters, because of all they've done to affirm their position and guarantee what they believe is a healthy profit.
With most companies, we not only have to wonder how they earn or plan on earning money (Instagram! Quora! Facebook!); their business plan may not even suffice.
Tumblr is currently twisting and turning to monetize their platform more, because David Karp has refused to use a business model that is implemented at the expense of users. But good intentions don't guarantee a (sizeable) profit.