You may also want to check out http://compete.com.

If you can find a site that released their numbers, you can compare that site (your baseline) with other sites to get an idea of how popular they are, relative to your baseline.

While you won't be able to actually find out how many users each app has (unless they release their numbers), this can at least give you an idea of popularity. Hopefully.

I'm thinking I could use twitter followers or alexa rank as a proxy for number of users. Anyone know where I could find a list of SaaS offerings?

And to also answer my own query, programmableweb.com seems to be the best resource.