I had a similar experience at LinkedIn in its early days. Back when we had maybe 500k members, we A/B tested the hell out of our invite messages. These messages were critical because no one had heard of LinkedIn at the time (~2004ish) and no one knew what social networking was or why they would want to sign up. And getting new users to accept invites was incredibly important to us. We A/B tested many invite email formats:
* A paragraph explaining social networking and another paragraph explaining LinkedIn.
* A few paragraphs on the benefits of joining LinkedIn.
* A few paragraphs explaining that a LinkedIn invitation was special because it meant that the inviter trusted and respected the invitee and wanted to stay in touch professionally.
* A tiny blurb along the lines of "I’d like to add you to LinkedIn network."
Despite my expectations that one of the explanatory messages would be best, the short, vague blurb outperformed by a huge margin.