SBI is an awesome initiative, and also a great example of the "pay it forward" ideology: You can't sign yourself up, you have to gift someone else in. I really like that. And yet? It's also a great example of what some might call "selfish selflessness:" I know there are a handful of people who have built pretty large share balances as a result of gifting dozens and even 100's of people into SBI.
The other beauty of this program is that it is definitely oriented towards long-term "players." As I recall, it takes about 115 posts to "break even." Which is excellent if you're building a long term presence in the community, not so good if you're trying to make money for next week's pizza!
The long term approach that is required keeps down the spammers from joining as they are looking for quick hit returns and may not keep an account more then a few weeks so no ROI for them. That is a good thing, F spammers.
Think with the increase in voting power everyone is seeing right now it might be a little less then the 115 posts to break even, but either way it takes some time. But it's instant ROI for contest winners!