Analysts and investors alike have zeroed in on Twitter’s user growth and engagement, key metrics for a not-yet-profitable company whose entire pitch is based on potential future earnings.

Regulatory filings show the number of monthly active users on the site is still growingup 39 percent in the third quarter from the prior yearalbeit at a slower rate. And advertising on Twitterwhich represents 89 percent of the company’s revenue as of Septemberstill relies upon users interacting with promoted content.

(Read more: What’s your Twitter IQ?)

But more than half of Twitter users, the poll shows, don’t engage on Twitter, they just read, or “lurk.”

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