For many, Facebook has become an indispensable tool for managing their social lives. But all the friending, messaging and poking on the online social network has created a hazard: using it too much.

Elizabeth Coe found out after she sent a link to her company's Web site to 100 friends and professional acquaintances. She got booted.

Through an e-mail a few days after she sent the link, Facebook said her account had been disabled for "persistent misuse of the site."

Facebook, she concluded, thought she was spamming her friends. "All I was doing is using it to communicate more efficiently, which is what I thought it was for," said Coe, 25, of Centreville, Va. "I don't feel like I was violating any code of ethics."

Others have been kicked off the popular site for adding too many friends at once; sending too many messages; joining too many groups; or "poking" too many friends, a casual greeting on the site. Shunned Facebookers said the punishment contradicts the site's core mission — to help people connect and communicate.

"Facebook is shutting down accounts of users who are exhibiting any behavior it finds remotely suspicious," Thor Muller wrote in a post called "13 Reasons Your Facebook Account Will Be Disabled" on www.GetSatisfaction.com, which offers customer-service advice. "As paradoxical as it sounds, 'suspicious' often means just using the site too much!"

A large part of Facebook's appeal comes from its ability to connect casual or distant acquaintances, such as high school classmates or friends of friends, said Matthew Salganik, assistant professor of sociology at Princeton University. But with 100 million users, shutting down accounts for questionable activity has become increasingly important. The network faces the challenge of allowing members to communicate when and how they want without inundating one another with a flood of messages.

Facebook's success also means that it must combat a growing amount of spam, bogus links and hoax messages, some propagated by malicious software that make it look like such messages are coming from friends.

About 64 large-scale spam attacks have been reported on social networking sites over the past year, and 37 percent of users have noticed an increase in unwanted messages in the past six months, according to Cloudmark, a Web security company.

Many spam attacks bombard hundreds of unsuspecting users with identical messages. So if a user sends a legitimate message or link to dozens of friends during a short period of time, he or she may be flagged by Facebook as a potential spammer.

Because of the rise in spamming attacks, including several incidents last week, Facebook has tightened its security systems and is deactivating accounts for behavior that seems at all suspicious, said Brandee Barker, the company's director of corporate communications.

"Accounts may have been deactivated not necessarily because of their activity, but because of the precautions we've taken," she said. Users who have been disabled will be reinstated on the site by e-mailing the company if they prove they've done nothing wrong, she said. "Because of recent security incidents, we've been overly cautious. We are working as quickly as we possibly can" to reinstate legitimate users.

Barker said users should avoid sending friend requests or identical messages to a lot of people at once. If a user needs to communicate the same message to more than 20 people, he or she should start a group rather than send individual messages, she said. While she declined to give specific limits, she said a general rule of thumb is to "go slowly with everything you do on the site."

Harry Joiner invited his Google e-mail contacts — all 4,600 of them — to join his friend network on Facebook. He, too, received an e-mail saying his account had been disabled.

The company's action "was very swift and very severe," he said. "All I did was follow the rules."

Joiner, a marketing recruiter in Atlanta, said it took three months for his account to be restored. He now conducts a lot of business on Facebook, connecting potential hires with employers. If he sends too many messages about a job posting, for instance, he runs the risk of being removed again.

Lisa Shane used Facebook to organize her high school reunion. Repeatedly sending the same messages to more than 200 people raised a red flag for Facebook.

A week before the reunion, she lost access to her account, as well as her contacts; RSVP list; and details about the venue, which she'd also arranged on Facebook.

"More and more people are using social networks to keep up important work contacts and to put together huge events," said Shane, of Baltimore. "If I can't count on that information being there when I need it most, just for using Facebook for its intended purpose, it makes me wary of using it to build contacts."