Here is a list of users sorted by reputation score on Parenting for all time.
We can see several people who do not post here any more. They do not ask questions. They do not answer questions. Some of them have high reputation scores, and a number of badges, and they would seem to be experts in some relevant topics. It's a shame that they stopped posting and the site would be better with them here.
Some amount of churn is natural.
So, Do we have a problem with user retention?
Mostly the site is doing well - but more users and more activity would help reach the tipping point needed to leave beta.
Here are some relevant questions. They don't answer this question directly, but I think it's all connected. They show that participation probably is a problem - we need more people asking questions; more people voting on questions and answers; and probably more people answering questions. But they don't address user retention.