The first e-mail I ever received from them was last week. It also sent the exact same e-mail twice.

Eh. Gmail collapses the duplicates for me, so I barely noticed. And you are being notified...

(Also, you've only used PB since last June or so, while it's been running since October 2009.)

the UI is awful

It seems pretty straightforward to me.

the site is often quite slow

That was much improved after Trike did the SQL profiling.

and their statistics package is quite rudimentary.

Yes, because varying proper scoring rules are why no one is using it?

There is no filtering mechanism for determining which predictions are "serious" -- the result is that many people post public predictions that should be private, but aren't.

This only affects Happenstance, not recording your own predictions.

All of these are annoying to various extents, but do they really explain the near-zero uptake?