Normally walled gardens are a user turn-off, however...
Recently the social networking site Facebook changed their membership policy. It used to be that you had to be a college student (or alum with college .edu email address) to participate. Last fall they introduced a high school program, presumably because they were jealous of MySpace's incredible success, primarily among high school students.
The high school program operated separately from the college program until February 27th, when Facebook started allowing college students and high school students to add each other as "friends." That's created a problem as Danny Shea and Matt Feinstein explain in their open letter to Mark Zuckerberg (founder of Facebook) published March 9th in the Daily Princetonian.
Suddenly, we had to begin removing tags from photos of us drinking, erasing wall postings referring to awkward hookups and getting rid of anything else that might negatively influence younger siblings or get back to once-adoring high school teachers.
Unfortunately, they don't understand that by posting "OMG how are you? I haven't seen you since our Model UN trip three years ago!" they are undermining the college personas that we have so carefully constructed over the past three years.
So, while walled gardens instituted by a service provider generally work against community formation (the service provider's customers are not usually a natural community), there are evident advantages to walls around specific communities.