Summer sales and ridership numbers show Citi Bike, at last, is on the rebound.

The bike-share program grew by leaps and bounds as New Yorkers embraced it immediately after the May 2013 launch, but before long, subscribers grew frustrated with unreliable service caused by buggy software and other operational problems. Sales and ridership slumped.

In fiscal year 2015, which ended June 30, Citi Bike annual memberships fell to 73,369, down 21 percent from the year before, according to the Mayor’s Management Report. The total number of trips also fell to 8.8 million, down from 9.4 million. City Hall attributed the declines to “harsh winter weather” and a jump in annual membership fees, from $95 to $149 last October.

Now, upgrades under new ownership — including back-end software fixes, a redesigned bike, a new docking mechanism, and app upgrades — appear to be paying dividends. Since July, ridership and subscriptions have been turning around. The size of the system has also been growing, but the positive trends predate the addition of stations.

In July, before stations were added, ridership hit a daily average of 35,960 trips, a 5 percent increase over July 2014. Citi Bike also sold more day and week passes this July than last July — a healthy sign.

In August, Citi Bike began adding new stations, a process that’s still underway. The growth has helped boost ridership to record highs. In the peak months of July, August and September, Citi Bike’s highest daily ridership in 2013 was 44,284. That number fell to 41,996 in 2014. Now, that number has reached a new benchmark: Citi Bike reports hitting more than 50,000 trips per day in mid-September.

It’s too early to say how the system expansion above 59th Street is faring, however. On the Upper West Side and Upper East Side, the stations aren’t as close together as the rest of the system, which threatens to degrade the user experience and curtail ridership. There are still about a dozen stations in those neighborhoods that are supposed to be added this year but haven’t gone in yet, making it impossible to assess how the thinner station network is performing so far.