Teens may love Twitter now, but the minute their mothers sign up, they're taking the conversation offline. And that should have Twitter worried.
Teens are a key demographic for social networks because they are an index for "cool" and can influence virality, said Amish Jani, managing director at FirstMark Capital.
(Read more: What companies are doing with your intimate social data )
"You can build a very successful service without teens [e.g., LinkedIn], but they can be a leading indicator for headroom in new-user acquisition," Jani said.
Twitter risks losing its edge, however, if it makes the same mistakes Facebook did, some teenagers say.
(Read more: With teens in mind, Facebook pushes both ways on privacy )
Students in the investment club at Warwick Valley High School in Warwick, N.Y., told CNBC they would stop using Twitter for two primary reasons: One, if Mom and Dad start poking around on the social network; and two, if annoying ads start appearing in their feeds.
For teens, social media is all about saying whatever they want to whomever they want, and they're not about to let the rents crash their party. The minute parents join, the fun is over, the students said.
"We [high school students] went from MySpace to Facebook to Twitter. I think when the parents really start coming into Twitter, I don't see myself being on it as much," said D.J. Mendenhall, a senior at Warwick Valley.
And if you're thinking teens can just block their parents, think again.