Enlarge By Karen Bleier, AFP/Getty Images The home page for Internet telecom site Skype during the outage. SAN MATEO, Calif.  The worst outage in Skype 's seven-year history ended Thursday morning after a technical glitch plunged it into darkness most of Wednesday. Millions of users of the Internet-phone service weren't able to make video and audio calls, and exchange instant messages Wednesday —Skype's most prolonged service interruption since it went down one day in 2007. Then, however, it had far fewer members. By noon Eastern time Thursday, things had improved. Skype has "seen evidence of a significant increase in the number of people online," Skype spokesman Peter Parkes says. Skype is estimating 17 million people, about 80% to 85% of its normal traffic this time of day, are using the service now. "The root cause was software," Skype CEO Tony Bates said in a phone interview Thursday. "We had to temporarily bring in some dedicated servers and shift resources to get back online." VIDEO: Skype CEO explains what happened Bates said Skype will offer users credits for the service interruption. Though highly unlikely, he did not rule out the possibility the service was the victim of a malicious digital attack. "We are really apologetic for what happened," says Bates, who took over as CEO just eight weeks ago. Some Skype features may not work as reliably as usual, however. Users' online status may be slow to update, and instant messages might not be delivered as quickly as normal. Other features, such as group video calling, will take longer to return to normal, spokesman Parkes says. The day-long outage comes at the worst possible time for Skype, which says this time of year is the busiest for video and audio calls over the Internet. With 25 million concurrent users a day — 140 million to 150 million unique visitors a month — it is aggressively expanding its customer base beyond PCs and smartphones. It has designs on tablets and televisions, as evidenced by deals with the likes of Samsung and Panasonic that put its products into some 40 million TV sets. Separately, Twitter says it is up and running for all of its 160 million-plus members. The social-media site was briefly down for some of its users yesterday. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more