SAN FRANCISCO — Alex Payne, a 24-year-old Internet engineer here, has devised a way to answer a commonly asked question of the digital age: Is my favorite Web site working today?

In March, Mr. Payne created downforeveryoneorjustme.com, as in, “Down for everyone, or just me?” It lets visitors type in a Web address and see whether a site is generally inaccessible or whether the problem is with their own connection.

“I had seen that question posed so often,” said Mr. Payne, who perhaps not coincidentally works at Twitter, a Web messaging and social networking site that is itself known for frequent downtime. “Technology companies have branded the Internet as a place that is always on and where information is always available. People are disappointed and looking for answers when it turns out not to be true.”

There is plenty of disappointment to go around these days. Such technology stalwarts as Yahoo, Amazon.com and Research in Motion, the company behind the BlackBerry, have all suffered embarrassing technical problems in the last few months.

About a month ago, a sudden surge of visitors to Mr. Payne’s site began asking about the normally impervious Amazon. That site was ultimately down for several hours over two business days, and Amazon, by some estimates, lost more than a million dollars an hour in sales.

The Web, like any technology or medium, has always been susceptible to unforeseen hiccups. Particularly in the early days of the Web, sites like eBay and Schwab.com regularly went dark.

But since fewer people used the Internet back then, the stakes were much lower. Now the Web is an irreplaceable part of daily life, and Internet companies have plans to make us even more dependent on it.

Companies like Google want us to store not just e-mail online but also spreadsheets, photo albums, sales data and nearly every other piece of personal and professional information. That data is supposed to be more accessible than information tucked away in the office computer or filing cabinet.

The problem is that this ideal requires Web services to be available around the clock — and even the Internet’s biggest companies sometimes have trouble making that happen.

Last holiday season, Yahoo’s system for Internet retailers, Yahoo Merchant Solutions, went dark for 14 hours, taking down thousands of e-commerce companies on one of the busiest shopping days of the year. In February, certain Amazon services that power the sites of many Web start-up companies had a day of intermittent failures, knocking many of those companies offline.

The causes of these problems range widely: it might be system upgrades with unintended consequences, human error (oops, wrong button) or even just old-fashioned electrical failures. Last month, an electrical explosion in a Houston data center of the Planet, a Web hosting company, knocked thousands of Web businesses off the Internet for up to five days.

“It was prolonged torture,” said Grant Burhans, a Web entrepreneur from Florida whose telecommunications- and real-estate-related Web sites were down for four days, costing him thousands of dollars in lost business.

Web addicts who find themselves shut out of their favorite Web sites tend to fill blogs and online bulletin boards with angry invective about broken promises and interrupted routines.

The volatile emotions around Web downtime are perhaps most prevalent in the discussion around Twitter, on which users post updates on who they are with, where they are, and what they are doing.

According to Pingdom, a Web monitoring firm, Twitter was down for 37 hours this year through April — by far more than any other major social networking Web site.

Instead of simply dumping the service and moving on with their lives, Twitter users have responded with an endless stream of rancor, creating “Is Twitter Down?” T-shirts, blog rants and YouTube parodies, and posting copies of Twitter’s various artfully designed error messages.

“This is a free service. It’s not like anyone’s life is depending on Twitter,” said Laura Fitton, a consultant and self-described passionate Twitter user.

“Twitter is all about the things we discover we have in common, so right there, Twitter failing is a huge thing we have in common,” she said. “It’s fun to complain to each other and commiserate.”

Twitter has said its downtime is the result of rapidly growing demand and fundamental mistakes in its original architecture.

Jesse Robbins, a former Amazon executive who was responsible for keeping Amazon online from 2004 to 2006, says the outcries over failures are understandable.

“When these sites go away, it’s a sudden loss. It’s like you are standing in the middle of Macy’s and the power goes out,” he said. “When the thing you depend on to live your daily life suddenly goes away, it’s trauma.”

He says Web services should be held to the same standard of reliability as the older services they aim to replace. “These companies have a responsibility to people who rely and depend on them, just as people going over a public bridge expect that the bridge won’t suddenly collapse.”

By some measures, despite the high-profile failures, the Internet is performing better than ever.

“There are millions of Web sites and billions of Web pages around the world,” said Umang Gupta, chief executive of Keynote Systems, which monitors companies’ Web performance. “These big high-visibility problems are actually very rare.”

But perhaps they are not rare enough. One morning last month, Google App Engine, a service that lets people run interactive Web applications, was unavailable for several hours.

Among those affected was Mr. Payne, who had just shifted downforeveryoneorjustme.com over to Google’s servers. It was inaccessible as well.

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vigilant is the price of freedom.