Directory enquiries <a href=" http://glanna.se/is-there-a-generic-effexor-xr-in-the-us.pdf ">mirtazapine venlafaxine to treat depression</a> In the laundry list of risk factors that's typically appended to all company IPO filings, Twitter warned it was heavily reliant on advertising revenue. It said more than 87 percent of its revenue came from advertising in the first half of 2013.