Everyone has 20% SP to "mine" everyday. You need to make 5 comments a day and upvote them. In order to do that, you need quality comments yourself, and those need to be equally rewarded. There was a time when you bought expensive hardware to mine, now you do it by participating. Of course, there is the equivalent investment for what once was hardware now to be made in SP. And the rest? They come and go. This post was invaded BTW.
Cheers