if you look at most product startup they all have started with shady growth tactics (stealing contacts or propagating trought fake invites to your contacts or pretending you to be on a network to lure your friends)

of course once they did it they where quick to plug the hole and denounce it as unhetical

spamming of course isn't a cost effective solution to reach people needing a product, but social network propagation trough referral still is highly effective

you can also buy targeted mailing addresses from selected companies, but the big, accurate lists are costly to come by - but those give you an incredibly focused group