Is MongoDB the right DB for every size of startup? What about the people who just want to try different projects on 1 (one!) single server?

What I've read about MongoDB, it wants to use all the memory it gets. So it's only usable if it is running on its own server. (And in the beginning of MongoDB it had to be 2 servers if you didn't want to lose data.)

Is this a false information again? Am I falling for anti MongoDB FUD again?