The friend had died in April.

It kind of freaked me out a bit, she said. It was like he was coming back from the dead.

Facebook, the worlds biggest social network, knows a lot about its roughly 500 million members. Its software is quick to offer helpful nudges about things like imminent birthdays and friends you have not contacted in a while. But the company has had trouble automating the task of figuring out when one of its users has died.

That can lead to some disturbing or just plain weird moments for Facebook users as the site keeps on shuffling a dead friend through its social algorithms.

Facebook says it has been grappling with how to handle the ghosts in its machine but acknowledges that it has not found a good solution.

Its a very sensitive topic, said Meredith Chin, a company spokeswoman, and, of course, seeing deceased friends pop up can be painful. Given the sites size, and people passing away every day, were never going to be perfect at catching it, she added.

James E. Katz, a professor of communications at Rutgers University, said the company was experiencing a coming-of-age problem.

So many of Facebooks early users were young, and death was rare and unduly tragic, Mr. Katz said.

Now, people over 65 are adopting Facebook at a faster pace than any other age group, with 6.5 million signing up in May alone, three times as many as in May 2009, according to the research firm comScore. People over 65, of course, also have the countrys highest mortality rate, so the problem is only going to get worse.

Tamu Townsend, a 37-year-old technical writer in Montreal, said she regularly received prompts to connect with acquaintances and friends who had died.

Sometimes its quite comforting when their faces show up, Ms. Townsend said. But at some point it doesnt become comforting to see that. The service is telling you to reconnect with someone you cant. If its someone that has passed away recently enough, it smarts.

Ms. Purvin, a 36-year-old teacher living in Plano, Tex., said that after she got over the initial jolt of seeing her friends face, she was happy for the reminder.

It made me start talking about him and thinking about him, so that was good, she said. But it was definitely a little creepy.

Facebooks approach to the deaths of its users has evolved over time. Early on it would immediately erase the profile of anyone it learned had died.

Ms. Chin says Facebook now recognizes the importance of finding an appropriate way to preserve those pages as a place where the mourning process can be shared online.

Following the Virginia Tech shootings in 2007, members begged the company to allow them to commemorate the victims. Now member profiles can be memorialized, or converted into tribute pages that are stripped of some personal information and no longer appear in search results. Grieving friends can still post messages on those pages.

Of course, the company still needs to determine whether a user is, in fact, dead. But with a ratio of roughly 350,000 members to every Facebook employee, the company must find ways to let its members and its computers do much of that work.

For a site the size of Facebook, automation is key to social media success, said Josh Bernoff, an analyst at Forrester Research and co-author of Groundswell: Winning in a World Transformed by Social Technologies.

The way to make this work in cases where machines cant make decisions is to tap into the members, he said, pointing to Facebooks buttons that allow users to flag material they find inappropriate. One way to automate the Is he dead problem is to have a place where people can report it.

Thats just what Facebook does. To memorialize a profile, a family member or friend must fill out a form on the site and provide proof of the death, like a link to an obituary or news article, which a staff member at Facebook will then review.

But this option is not well publicized, so many profiles of dead members never are converted to tribute pages. Those people continue to appear on other members pages as friend suggestions, or in features like the reconnect box, which has been spooking the living since it was introduced last October.

Ms. Chin said Facebook was considering using software that would scan for repeated postings of phrases like Rest in peace or I miss you on a persons page and then dispatch a human to investigate that account.

We are testing ways to implement software to address this, she said. But we cant get it wrong. We have to do it correctly.

The scanning approach could invite pranks as the notification form already has. A friend of Simon Thulbourn, a software engineer living in Germany, found an obituary that mentioned someone with a similar name and submitted it to Facebook last October as evidence that Mr. Thulbourn was dead. He was soon locked out of his own page.

When I first died, I went looking around Facebooks help pages, but alas, they dont seem to have a Im not really dead, could I have my account back please? section, so I opted for filling in every form on their Web site, Mr. Thulbourn said by e-mail.

When that didnt work, Mr. Thulbourn created a Web page and posted about it on Twitter until news of the mix-up began to spread on technology blogs and the company took notice. He received an apology from Facebook and got his account back.

The memorializing process has other quirks. Memorial profiles cannot add new friends, so if parents joined the site after a child died, they would not have permission to see all the messages and photos shared by the childs friends.

These are issues that Facebook no doubt wishes it could avoid entirely. But death, of course, is unavoidable, and so Facebook must find a way to integrate it into the social experience online.

They dont want to be the bearer of bad tidings, but yet they are the keeper of those living memories, Mr. Katz, the Rutgers professor, said. Thats a real downer for a company that wants to be known for social connections and good news.