It's as simple as this: it is extremely hard to do a big business on something that is trivially reproducible (like Twitter is), just because there is a big momentum at some point. Because the outcome is one of the following three possibilities:

1) You ruin the experience because of the business model. People switch to a competitor that is annoyances free as you were.

2) You ask for money. People switch to a competitor that is free as you were.

3) You invent a business model that is an added value for users instead to be a problem. You win.

To make "3" working you need to be open minded and design the business model for months, with creativity, thinking at your users. It's hard but you could do it, but unfortunately there are this guys that gave you millions that will ruin this process. So "3" is very very very hard for Twitter IMHO.