The correct interpretation is:
Ninety-five percent of the time, the procedure used to generate this interval will capture the true proportion of teenagers who use Mybook as their primary social network site.
This means that if we were to take many random samples of teenagers and construct a confidence interval using the same procedure, we would expect that 95% of those intervals would contain the true proportion of teenagers who use Mybook as their primary social network site. It does not imply that 95% of all samples of this size would yield a confidence interval of (0.2964, 0.9036) specifically.