Use btcjam if you are a long term investor. Otherwise ponzi schemes like hyip will be nice if you afford to loose bcoz buying bitcoins itself a gamble
I think you did not read the topic or what OP is saying that is why you are suggesting btcjam to him, because btcjam is a loaning service which would require you to invest quite a large capital in that so that you can get some good interests on that, but 100$ is not that much, it is good for trading which can earn you some good profits in a short time with that amount.