"Is Facebook going to be the frat party after the last keg has run dry?" asked eMarketer's Williamson, voicing the argument she's heard bandied about.
But though younger users are known for their fickleness, Williamson said this hasn't happened yet and she doesn't expect it to happen soon.
Rob Enderle, an independent analyst, said it depends on how Facebook manages the different demographics and customizes their experiences.
"If you can segment the demographics and serve both groups, it isn't a problem," he said, pointing out that Facebook has recently started to move in this direction by giving users the ability to assign friends to different groups depending on the nature of their relationship (personal, professional, familial, etc.).
At some point, he and Williamson expect Facebook to equip users with tools to send out (and restrict) communications based on these groups. For example, a picture from a college reunion would go only to college friends while a message about a work event would only go only to professional contacts.
"I'm not seeing people abandon Facebook," Enderle said. "Once you put a lot into it, you don't want to duplicate that someplace else."
And though much has been made about young members worrying that their parents might friend them to spy on them (a Facebook group, "For the love of god -- don't let parents join Facebook" exemplifies that fear), many parents and users over 55 say they joined precisely because their children or younger relatives asked them to.
Debbie Donaldson, a 57-year-old from Sonoma County, Calif., said she joined about a year ago at the behest of her 23-year-old daughter.
"My initial reaction was this is just for kids," she said. "But my daughter said there's more of a sophistication to it."
Now she uses Facebook regularly to keep up with her daughter and her friends, and to reconnect with her own friends and former classmates. But she joked that older folks have a tougher time finding friends for one key reason: Hoping to hide creeping wrinkles and added weight, "the old people, they're not putting their pictures on their pages."
Though Donaldson enjoys the online interactions with her daughter, being her "friend" on Facebook is not without its complications.
Although she'll comment on pictures posted by her daughter and her friends, she said she's very careful about what she says.
"I don't want to sound like I'm one of them," she said. "I don't want to be less of an authoritarian than I am."
Another parent, who asked to remain anonymous, raised similar concerns about the new "etiquette code" on Facebook.
When you friend a child's boyfriend or girlfriend and they break up, do you automatically un-friend the ex? Or, if your child un-friends a mutual friend after an argument, do you also remove the friend out of solidarity or remain neutral and do nothing?
For their part, the children of these older users acknowledge that finding parents -- and in some cases, grandparents -- on the site has been a bit surprising. But they also recognize that the site itself is still evolving.
"When he first joined, of course it was a little odd," said Claire Buchignani about her father's foray onto Facebook. "It was my dad on Facebook, and originally it was just for students."
But, she said, "I think that there's definitely enough room for all of us."