He was hired very recently, shortly before the acquisition.
TVD is one of the "superstar" web designers, frequent speaker at various designer conferences, with a huge following on Dribbble. He himself is notable, but his contributions to Instagram are much less so.
I'm not so sure about that – since he was hired, most screens and nearly all the Instagram UI icons have been refreshed. He's literally all over the app's UI right now.
Certainly, a refesh. He was not instrumental to defining Instagram as a recognizable product though, that's my point. He's a great designer, but labeling him as "of Instagram" is unfair to both him and Instagram's original designers.
TVD is one of the "superstar" web designers, frequent speaker at various designer conferences, with a huge following on Dribbble. He himself is notable, but his contributions to Instagram are much less so.
[1] http://dribbble.com/maxvoltar