XM Satellite Radio

(XMSR)

is still tuned in to growth.

The Washington, D.C.-based pay radio broadcaster says it added 640,000 net new subscribers in the second quarter ended Thursday. The new-user total represents a blowout growth number exceeding nearly all analysts' expectations.

Analysts were looking for XM to have signed up 545,000 new users during the period, roughly matching the company's strong first-quarter numbers.

"This was the best second quarter in the history of XM. The momentum behind XM's subscriber growth is undeniable," says CEO Hugh Panero in a press release Friday.

XM has doubled its subscriber base in one year and now has 4.4 million users. The company did not give any new forecast for its year-end total.

Shares of XM jumped 62 cents, or 2%, to $34.28 in late-morning trading.

Observers note that in the past, XM's strong subscriber growth numbers usually triggered a little selling on the day of the announcement, in large part because the stock had risen in anticipation of the news. XM shares have jumped 21% in the past two months.

In May, XM surpassed the 4-million-subscriber mark and stuck with its earlier projection of having more than 5.5 million users by year-end.

At last count, rival

Sirius

(SIRI) - Get Report

had 1.5 million users and expected to hit the 2.7-million-subscriber mark by the end of this year.