I don't think I've seen the same degree of this with Microsoft before. Is the difference that FB and Google are both in Silicon Valley and so there's very low friction moving from one to the other?
http://en.wikipedia.org/wiki/Non-compete_clause#California
The article mentions a case in 1998 where out of state non-compete clauses cannot be enforced in California.
They are enforceable in Washington state, Microsoft's main location:
http://en.wikipedia.org/wiki/Non-compete_clause#Washington
This might be one reason why you haven't seen something like this at Microsoft.
More recently a lot of Microsoft people have gone to Amazon or Google (which has done a lot of hiring in Seattle and for a while was aggressively recruiting MS folks) ... it'd be interesting to see how the numbers compare with FB/Google crossover.