2 startups is hardly enough evidence to say that's the only way of acquiring users.
Besides, if the "spam" is actually helpful and appropriate, what is spam to you may be a welcomed way to learn about a new product for another person.
That said, never trust a marketing messaged signed by the Prince of Nigeria. ;)
Who did Instagram spam? Would you rather have been Instagram or Path?
Wait, I thought AirBnB called it 'growthhacking'.
User acquisition is probably the biggest challenge for an early-stage startup (http://www.paulgraham.com/growth.html). Your definition of "spamming," is pretty broad.
"Spamming" by itself is not malicious. In the case of Path, the app is only as valuable as my network of friends who is on it, so you can be damn sure I'm going to invite them. In the case of AirBnB, people saw the ads, signed up and ostensibly got value out of AirBnB, so what was the harm?
Not sure what your problem with "spamming," is (even though you have provided two examples that are pretty different), but I imagine that yes, startups will continue to grow at all costs because they have to in order to survive.