He deleted his profiles on both social networks and started a new one on Facebook under a different name, only letting his closest friends know how to find him. An alter ego was born.  "All of the sudden I felt completely exposed," said Mitchell, a 22-year-old financial analyst in the District. "It was fine when I was in school, but I just started my first job. I can't be out there like that."  Other people in their 20s and even teenagers are doing the same, assuming online aliases on such sites as Facebook and MySpace to avoid the prying eyes of parents, college recruiters, potential employers and other overly interested strangers. They are also being more selective in who they allow in as "friends" by paring back the size of their social circles.  Social networks, which let members share photos, videos and intimate details about themselves and their friends, have pushed the boundaries of how people view their personal space. Now, the younger generations that used to embrace the voyeuristic qualities of the Web are considering the advantages of borders between their public and private lives.  In person, people tend to adapt their behavior to the situation -- talking to a co-worker requires different language and attitude than what's comfortable with a college friend. On social networks, everyone's in on the same conversation.  Not only that, an indiscreet comment in a face-to-face exchange can be regretted and forgotten. Online, it can live for years, providing personal details to potential bosses and marketers.  "For the first time in history, we can't tailor our image for our specific goals," said Mark R. Leary, professor of psychology and neuroscience at Duke University. "When we have to create an all-purpose social history, how do people juggle competing audiences?"  Modifying online personalities in search of more privacy "is a natural evolution in our relationship with these technologies," said Mary Madden, senior research specialist with the Pew Internet &amp; American Life Project, who has studied privacy issues among teens on social networks. "There's a bigger pool of people to think about now. Networks we initially considered to be curated and private can start to feel very public."  Pew research has shown that 66 percent of teens with an online profile have restricted access to it. Among teens who allow their profiles to be seen by others, nearly half provide some fake information.  Barbara Leary (no relation to the Duke professor) joined MySpace two years ago, but when she entered high school this year at School Without Walls in the District, she scrapped her account and started over. Leary, 15, no longer posts her full name and feels less obligated to accept every friend request she receives. When she opened a Facebook account this year, she used the strictest privacy settings available.  "I started using MySpace when I was a seventh-grader and wasn't as careful," said Leary, whose mother has suggested that she sprinkle fake information into her accounts. "Now I'll be friends with someone if I see I have a lot of friends in common with them. But if I don't know them, I don't let them in."
Since joining Facebook and MySpace two years ago, Luke Mitchell has amassed 476 friends, mostly buddies from high school and college. It was a great way to keep in touch, until his boss showed up on the sites and saw the embarrassing party pictures a friend posted.