Twitter's problem is that they invented a great communication channel with no clear way of monetizing it.
Like inventing email.
They're now trying to build walls around their protocol, shutting down APIs, etc. Imagine if the people behind email would have done it.
For me, Twitter is a great news reader. And Google shutting down their Reader product should explain the problem with this kind of products.
It's littered with sponsored tweets and advertisements EVERYWHERE.
Twitter analyzes your words and interests, your network, your locations, and sells you as a demographic to various businesses for targeted advertising.
They're pretty much an ads platform like Google Search or Facebook.
Email itself is not monetized. RFC 5321 charges no license fees. Platforms built on top of email are monetized.
I use Twitter daily, and would hate to see it disappear. But how much of current revenues are a product of companies still "experimenting" with the Twitter ad ecosystem? What happens if those channels provide less ROI than everyone expects? That's the way the trends appear to be heading.
Twitter's 'cost per impression' is much higher than Facebook's...might not make it as attractive to advertisers.
If Twitters CPM is much higher than Facebook's it's because it is attractive to advertisers. Otherwise they wouldn't charge as much as they do. (unless there's something else going on, which seems doubtful in this case)
Not necessarily. It may simply be overpriced but not yet at the point where the market has enough information to force a correction.
Email (which I assume is a reductionist placeholder for the sum of its protocols and implementations), to the best of my knowledge, was never built with "monetization" in mind, not more than any open standard or protocol is.