Sirius (SIRI) boosted its subscriber count in the latest quarter, but not as much as its fans on Wall Street had hoped.The satellite radio broadcaster's shares slipped 6% Wednesday, after a first-quarter financial report showed the company improving its top line but failing to make substantial inroads into its massive losses. The New York company added 90,602 new customers in the quarter ended March 31. That's a 35% gain, taking its subscriber count to 351,663 at the end of the first quarter. But Wall Street analysts had expected the company to expand its user rolls by something like 110,000 subscribers. Meanwhile, high costs and subscriber acquisition expenses translated to a net loss of $146 million, or 12 cents per share, on $9.2 million in sales. That compares to a loss of $148 million, or 14 cents a share, on $5 million in revenue in the previous quarter ended in December. Analysts had expected a 10-cent loss on $9 million in sales. The pay radio player increased its average monthly revenue per subscriber, or ARPU, to $9.92 from $8.59. And though the company trimmed its subscriber acquisition costs to $248 per customer from the $293 level last year, the expense is still much higher than the $200 that the company has predicted. Shares in Sirius and rival XM ( XMSR) surged last year as investors leaped aboard the satellite radio bandwagon. The companies charge $10 and up for monthly subscriptions to their mostly commercial-free radio broadcasts. The programming is broadcast from satellites to special receivers mounted in vehicles or homes. Bulls believe the medium has nearly unlimited growth potential, and seize on the companies' fast-expanding subscriber rolls as evidence. But bears note these outfits' cash-consuming ways and wonder if the business can ever expand to the point where either company, let alone both, is consistently profitable. Sirius shares slid 23 cents to $3.57. Meanwhile, XM dropped 11 cents to $27.38.