For points of reference, this violates Facebook's terms of service, Twitter's, and several of Google's, though not directly mentioned for Google+ I'd still err on the side of caution.

One of the things you can do to be more compliant is to offer an "opt out" option, or an alternative method (like a unique URL) that doesn't directly involve the social networks sharing plugins but still provide an effective gate. Just be aware that this will cause issues with at least three of these services. Can't speak for LinkedIn, but I'd assume it's probably the same deal.