It may be similar to a pyramid scheme in that those who join early will have the opportunity to make the biggest profit. But in a pyramid scheme, those who join early will continuously receive money from the lower part of the pyramid, whereas in Bitcoin there is no guarantee that the early adopters will get anything at all! Because if the system crashes, then even the those who joined early will lose unless they have managed to sell their bitcoins before the crash. So Bitcoin is more like investing in a growing company. Like investing in Facebook or Google when they were small companies. Or like investing in similar startups that went bankrupt. So there is a risk involved in the Bitcoin system in contrast to a pyramid scheme, plus the most important thing is perhaps that a pyramid scheme is just a fraudulent system leading to a certain crash while the Bitcoin system is something that builds to a potentially very useful monetary system with immense intrinsic social and real value.