“You’re watching other people enjoying your daughter and the grandchild you’re supposed to have, and you’re left out in the cold,” Mary said. “I have to watch pictures of my grandson — that I didn’t get — on my daughter’s sister-in-law’s page.”

Their rift also plays out in front of relatives and friends, in a humiliating way. When Mary’s 21-year-old son took his sister’s side and cut off contact for a few months, he not only removed her from his friends list but also disowned her on Facebook. “It was a blank little ghost where his face used to be on my profile,” she said.

Some parents said they could not help but check up on their estranged children online, searching for clues to unlock the mystery of why they have been cut off. If a parent’s child has blocked them on Facebook, they might send a “friend” request to their child’s acquaintances, follow them on Twitter or do Internet searches to find out where their grandchildren attend school.

“This is a common question on my Web site — whether it’s more painful to sneak views on Facebook or not, or just to stay away altogether,” said Elizabeth Vagnoni, a documentary filmmaker who runs an online discussion forum for parents with estranged children — currently with more than 2,200 members — at estrangedstories.ning.com. She went through bouts of silence with her mother over a decade, and now has two estranged sons of her own. “I now live with knowing exactly how I made my mother feel,” Mrs. Vagnoni said.

For those who hope to reconcile, the online sleuthing can backfire if it is perceived as spying, said Monica McGoldrick, a family therapist and the director of the Multicultural Family Institute in Highland Park, N.J. For a mother, she said, “If ever there comes a day where she and her daughter get to have a conversation, what is she going to say?”

People who use social media are often aware that their estranged relatives are watching. A decade ago, Jessica, a 27-year-old Manhattan resident who works for a marketing company, stopped talking with her father, an alcoholic, after much turmoil. (Shortly after he remarried when she was 10, she called him and said, “You can divorce your wives, but you can’t divorce your children.” His reply, she recalled: “I wish I could.”)

She sends Twitter messages for work and for fun, and knows through other relatives that her father is keeping up with her — her vegan diet, her travel schedule, her social life. It angers her, even though she acknowledges anyone can see her posts.