It really depends on what kind of features and user data you're accessing. If it's just for general authentication/identification, sure. On the other hand, if you'll be accessing their contacts, private data, posting to their feeds or anything like that, I strongly recommend not.
If you need to access the user's private data to test your service, be extra clear what you will be doing with it upfront. That way your testers can decide whether they want to allow you access to their personal accounts or create their own dummy accounts (if they really want to check it out and/or support you, but are uncomfortable with granting access to their data).