Honestly, if that's one of the methods one chooses to get a startup off the ground, I have no problem with it. It's a small moral price a startup has to pay to get some momentum going, and without initial momentum, you really can't go anywhere. Sure, it's an annoyance on the user end, but if it's a truly useful service, I see no problem. Every user, especially in the initial stages, is valuable.
I have to spend a not small part of my time dealing with various forms of spam, and dealing with people that hate dealing with spam, and dealing with people that hate missing out on something because somebody somewhere was dealing with spam on their behalf.
I guess it would be easy to say, "Eh, what's the big deal?" if you never have to deal with the cumulative effects of everyone promoting their products in spammy ways.
At the very least, it's disrespectful to the folks that don't get paid enough to keep all the networks running everywhere.
Well, that may be true. But from the perspective of one trying to gain a little traction, spamming might serve well, especially after hearing that the persons behind facebook and plentyoffish probably used the same tactics.
