The trend is still relatively confined. Only about 7 percent of people older than 65 have online social-networking profiles, according to research from the Pew Internet & American Life Project.

But Facebook's popularity is growing most quickly among women older than 55, according to a site called Inside Facebook, which tracks Facebook's growth.

There are now about 1.5 million female users older than 55 on the site, the group says -- roughly a 550 percent increase over six months ago. By comparison, membership among people younger than 25 grew by less than 20 percent over the same period, Inside Facebook says.