Their annual revenue is about $6 per user [1]. The problem isn't absolute failure to monetize, it's massive costs. If you thought you could run twitter without their ~4000 employees, you might have a chance of pulling some cash out of there.

The problem is, it's hard to simultaneously clamp down on costs and push for growth. In SV, anything other than growth is an admission of failure. If you aren't flinging money into R&D, everyone will assume it's the start of a downward spiral.

[That, and you can't buy twitter without outbidding all the True Believers]

[1] http://www.viewproxy.com/twitter/2016/2/annualreport2015.pdf