When meeting a referral prospect in a social way for the first time, it's preferable to have the referral source present — to make the "formal" introduction, say a few nice things about you and help keep the initial conversation going.
While this is ideal, getting three or four people to coordinate schedules is not always easy, and you want to be careful not to lose the momentum of the referral.
Most of the time, this social meeting is just that – purely social. You get the chance to engage with the prospect on a social level first and at the end of the encounter you can either arrange the "business meeting" or at least gain agreement to send the prospect an email to set it up. Whenever possible, get agreement to this meeting in the presence of the referral source. When the prospect makes the commitment to the next step in front of the referral source, they are much more likely to follow through.
If the referral source is not present, you may very well have some opportunity to talk about the prospect's situation related to the work that you do. The more you learn about the prospect from both your referral source (warm research) and the internet (cold research), the more meaningful and interesting your conversation will be.
Unless the prospect takes you down the path of a full-fledged business conversation, this social encounter should be mostly social (70 – 90 percent).
Set the frame
Your success in turning this social meeting into a business meeting is all about the expectations you establish, even before you meet. Some people call this "Setting the Frame."