This is an interesting one. The music of Placebo is certainly not goth, but it is goth friendly, much like the music of Kate Bush or the Eurythmics. Older goths would sciff at Placebo, of course, but most goths under the age of 25 have owned and liked at least one Placebo album at some point in thier lives.
As for the individuals, well, we've got some powerful plus points. Make up and ambiguous sexuality for instance; this is certainly more true of the earlier phases of Placebo. Given that no one in their might minds would try and argue that Robert Smith isn't goth* it could be hard for the layman to see why we can't come out and call Placebo goth. However, we have to look at the overall picture here. Problem is, Placebo are an NME band. No way round that, and the NME have never got goth. When Siouxsie and the Banshees, Joy Division and The Damned started hacking out the basic foundations of goth from the soft clay of post-punk back in the early eighties the NME tried to 'discover' the goth rock genre, and tried to call it 'positive punk', which really goes to show how fucking stupid they are. This association, at the end of the day, makes calling Placebo goth worryingly like calling Muse prog rock.
*Except Robert Smith, for obvious reasons.