I've been looking at VPS's and the specs they offer for entry level setups seems somewhat surprising to me. I'm am new to this topic but many of VPS offer less than 512MB of memory and my laptop has 4GB of memory so I am curious what does it actually take in terms of hardware to serve say 10k users (say 5k daily active users)?

I figure a large number of factors can probably sway this a lot but just for benchmarking, say the site is a social networking site written in php using mysql + apache that's not really doing anything unusual like serving lots of media. So essentially a very basic Facebook minus the absurd number of photos and videos. What about 100k users (50k daily active)? 1 million (500k daily active)? Thanks in advance.