It's an interesting question. By comparison, at the end of 2010 Facebook had about 2,000 +/- employees and $2b in sales for that fiscal year. Twitter is roughly three times overstaffed compared to Facebook.

A good portion are likely selling ads and supporting advertisers.

On some level, everyone on Twitter is promoting themselves, and grooming their followers to a particular image or persona.

Full disclosure: I used to work at Twitter, and this conversation came up all the time. I even proposed it myself as a naive new hire, and after a few good-humored suggestions from coworkers to run the math, ended up with roughly the following scenario:

Say 1% of Twitter's active users would be willing to pay for such a service. (That's extremely generous, I think, but it makes the math easier.)

At 240 million MAUs, that gives ~2.4 million paying users. Say the price was comparable to Flickr at $50/year. (Again, very generous, and probably out of reach of most users outside US/Japan/EU, but we'll try it for the sake of argument.)

After up-selling each and every one of those users, the company could have annual revenue of about $120 million...or roughly _half_ what they cleared in Q4.

Even with 2-3x organic growth in the next couple of years, it would be a dead-end in terms of revenue, and better-financed competitors would be able to push them around, poach their best staff, etc.

You may be interested to know that Twitter does offer such things: https://support.twitter.com/groups/58-advertising#topic_247