Answer: Facebook Messenger.
I get patients / leads to message my clients, then I use a 'bot' to create an automated, personalized conversation WITHIN messenger.
The conversation is actually a funnel sequence that gets warm prospects to take action and disqualifies irrelevant leads. It saves my clients time while increasing the number of good leads.
Why is it so effective now? These have a very high response rate, since people get notifications on their phones for new messages and always check it (compared to ~10-25% open rate in email).
I'll be writing more about this. Warning: Get on this while you still can, because as more marketers jump on the bandwagon, it'll get saturated and we'll see diminishing returns.