Twitter’s advertising rates fell again in the last three months of 2013, according to the company’s annual report released Thursday. The cost of advertising on Twitter dropped 18%, the seventh straight quarter of declines. The trend is worrisome for Twitter, but for now it isn’t a big deal that advertisers are paying less for each ad unit. Advertisers are overall spending more on Twitter because more users are clicking on ads. The engagement growth shows advertisers that ads on Twitter work, thus spurring more demand. It more than doubled to $220 million in the last three months of 2013 compared to a year earlier.