You can find an archive of the article here:
https://www.webcitation.org/mainframe.php
Thankfully, the NSA slides were directly included by wikipedia, so we don't have to hunt them down:
https://en.wikipedia.org/wiki/PRISM_(surveillance_program)#/...
We can see that PRISM targets (or should I say, partners) list contains at least: Microsoft, Yahoo!, Google, Facebook, YouTube, AOL, Skype Apple.
As usual, this is what we know. Meaning what could be extracted, proven, communicated and accepted by the public. A pretty big filter already.
#/media/File:PRISM_Collection_Details.jpg){kind=link}