I don't know!
This is a two sided marketplace, which is tricky.
Normally, you'd want to go after the 'long-term' side of the market first. For example, AirBnB would want to go after the homeowners first, because they get long term value from the platform so will be more willing to sign up even though there's no value today.
You don't have that though, because both sides of the market only have short-term value really (I need a flatmate now, not in 6 months).
Maybe there's some way to 'hack' this. To work with third parties who already have a list of people likely looking for flatmates.
Student organisations for example. Companies hiring lots of fresh graduates. Landlords...