I liked the idea and now I use this plugin to my gmail account (https://chrome.google.com/webstore/detail/followup-the-personal-crm/hgfakmfpmkaaamdjfolpjgdofepbfhke?hl=en) and I feel to restricted to gmail...
Following this idea:
Is the tool for professional or personal contacts?
Instead of a unlimited subscription model, have you ever thought about making it freemium according to the number os contacts a person add?
*For me this model will work better, because I don't have so many people on my personal network and I believe $5/month will be expensive.