I can't count how many times I've seen this debate. I think last week there was a thread debating the triviality of web apps and similarly mentioned Musk.
Elon Musk may very well have developed a cat picture social network to gain the wealth required to fund his companies, but he instead worked on X.com/PayPal. Do you really believe Musk would be where he is today if he didn't gain considerable wealth from that venture? He'd likely be drawing rockets in a sketchbook while working as VP of Advertising at Google.
The Instagram founders are only wealthy enough at this point to have great credit, who knows what great investments they could make post-close?
I'm a fan of taking the simplest, reasonable route to wealth, then investing in something of greater importance, something that critics would agree improves humanity.