Acquiring social networks that get to scale is an integral part of Zuck's strategy. It is a fairly smart strategy since one company cannot successfully lead or copy all the innovation - and social networks are very "taste-dependent" products. Zuck understands that Facebook is a big incumbent, and as with any incumbent there would be 'anti-incumbent' products. He was just willing to buy these products without too much fuss.
His willingness to overpay for these companies has been a part of a long-term dominance strategy. Maybe he has one or two misses like Snap; but by and large, his sense for "pricing" these companies and a no-nonsense "Corp Dev" approach could be thought of as a competitive advantage (plus he has proven that he can do it repeatably). The lawyers don't get involved until much later - he "leads" from the front when doing these deals. It is not very common in large-company-land.
If Jan Koum, who is very highly respected (he is truly a no-bullshit product/management person) leaves for the "wrong" reasons, it might have long-term implications for other companies that Zuck wants to buy in the future. Thus leaving an opening in the market for an 'anti-incumbent' offering to thrive.