I've seen people get really good results from even mailers with very small memberships. I think the important factor is freshness. How many fresh members are on that list? Or active members on that list... but judging by new members is easier. If a list only has a few hundred but they all recently joined because it's brand new, the click rates and results will still be very high... then in the future it will be up to the owner to keep them active and continue to bring in new users.