I've read that Alexa scores are not reliable metrics, but I also noticed that my site's score shifted drastically since I moved from a vb to xf. I had a vb forum up for about a year (with very few posts because the software was glitchy and not user-friendly). I moved from vb to xf a few weeks ago and I noted the score quickly shot from 750,000 to well over a million. I only recently installed google analytics so I can't say if there was actually a downward shift in traffic (but I doubt there was). I'm just curious whether or not this type of shift is unusual given the circumstances.
I wouldn't read too much into it. Alexa's metrics are way off for anything < 100K. There's just not enough traffic for them to get a decent sample size. At that level, just a few dozen visitors can hugely affect your rank.