I've seen one site that I know for sure is 'powered by Oracle': Oracle's
own Metalink site. Its performance is utterly appalling: half-minute
delays between doing anything and the response... Oracle's stunning (lack
of) useful full-text search capabilities shine through in the completely
hopeless search page as well. Of course I don't know what systems back
this site but I doubt it's exactly underpowered.
For a long time they had their advertising slogan 'Oracle Software Powers
The Internet' on there. This led to despairing laughter and the
occasional 'thank god it doesn't' from everyone who saw it, including
various Oracle employees.
Oracle is quite good at massive thumping bank systems, but I wouldn't back
a website with it if I were you. Even Oracle can't make that work.