Sounds interesting, but I don't think PHP & MySQL are a good stack. While it's possible to write secure software in PHP (I'd probably trust the Paragon IE) guys to do it), it's not very likely. As for MySQL — as far as I'm concerned there is essentially no reason to choose it over PostgreSQL, and a project which does so is thus instantly suspect. If they've made an error that grave, what other errors have they made?…
The LAMP stack, for all its problems (you're not wrong) is the bog standard and is easy to rent.
If you'd rather use Rails, check out Diaspora: the two systems federate together, anyway.