Most common advice is that you have to be at least one of the sides somehow. Reddit famously did this with lots of sock puppet accounts to foster discussions and create pseudo activity.

In your case, I'd probably start by reaching out to businesses in mid-size metro markets, ones where bike couriers don't already exist, and offer to save them on shipping small packages. Build up a list of clientele and encourage them to contact you for jobs when they need to ship small ad-hoc stuff. That should give you an idea of demand. Then start posting on craigslist and facebook looking for delivery drivers, then start match-making. From there encourage the drivers you find to sign up on your platform for future work.