SAN FRANCISCO (AP) — Twitter’s ad revenue is believed to be on pace to double this year as the online messaging service delivers more marketing pitches to mobile devices in preparation for a highly anticipated initial public offering of stock.

For now, Twitter remains a privately held company that staunchly refuses to disclose its financial results. That leaves it to research firms tracking Twitter to try to divine how well the 7-year-old company is doing. The estimates are based on a variety of sources, including market trends, ad agencies and other industry data.