I appreciate your efforts to make this site grow and attract new (or in this case old) users, but I feel you may be a little too enthusiastic.
First of all, Buddhism SE is doing a great job for a beta site of this age. Personally I think the question quality could be a bit better, but if I look at number of users, visits and questions, this site is doing great.
Now, if a user has decided not to visit this site anymore, he/she may have perfectly good reasons for this. There is no possibility for users to send messages to others users outside of the StackExchange website unless they've provided you with their email themselves (e.g. in their profile). This is intentional(!) because the last thing we want to do is annoy people by sending them unwanted emails.
You can attract the attention of a user that is still visiting StackExchange sites by typing @username (replace 'username' with their actual username and skip the spaces if there are any) in a comment or chat message. If you do this a notification of your comment/chat message will appear in that user's StackExchange inbox in the top bar.