Major sites including Reddit and Foursquare crashed or slowed Thursday after glitches hobbled Amazon.com's Web servers. Amazon, a cloud-computing seller as well as an online retailer, provides cloud services to numerous tech startups. Cloud computing is generally seen as more reliable than conventional hosting services because if one computer fails, the tasks it performs can be taken up by others. But Thursday, the Amazon data center near Dulles Airport outside Washington ran into trouble, bringing down many of the sites it hosts. HootSuite, which helps users monitor Twitter and social networks, was down completely, as was answer site Quora. Amazon has not commented on the problems.