From the "one less Mike at Mozilla" files:

Mozilla VP of Engineering Mike "Schrep" Schroepfer is leaving Mozilla and headed to Facebook - to serve as Director of Engineering. Mozilla seems to like guys named Mike in key roles - there is Schrep, Mike Beltzner, Mike Shaver and Mike Connor.

There is no shortage of smart guys (named Mike or otherwise) at Mozilla so I personally don't think that Schrep's departure will have any significant impact on Mozilla operations. Mozilla Firefox 3 is already out the door and Firefox 3.5 isn't coming till the end of the year.

On the other side, Facebook could sure use a guy of Schrep's expertise on a number of levels. Schrep knows how to make development decisions and  work with  a community of developers (internal and external).  So good luck  to you Schrep and good luck to whoever replaces you officially at Mozilla.