They can't easily afford to splurge on the best acquisitions at this point. Instagram cost $1 billion. YouTube cost $1.65 billion.
Snapchat is heading toward bankruptcy or a forced sale if they don't dramatically cut costs. They're down to $1.4b in cash (from $2b three quarters prior). They can't safely buy an Instagram at this point. Pretty soon they're going to be the $1-$2 billion acquisition.
Twitter is in much better financial condition of course, although a $1-$2 billion acquisition is still a huge chunk of change for their business ($6b in cash, ~$300m-$400m per year in profit). Twitter's problem is that their cash is precious, because their business doesn't generate a ton of it (they've been persistently hauling billions of cash around from their past funding days). That cash is a big safety buffer that they can't easily replace if they burn it. They can use their market cap of course, and shareholders will only tolerate so much of that at their size and given their non-growth context. Twitter needs to figure out what it wants to do before the market 'realizes' what they're actually worth (zero growth gets you an optimistic ~20 PE * $350m profit = $7 billion; a further 2/3 drop from here); it's similar to the old Yahoo scenario pre Alibaba (no growth, the market temporarily giving you an abnormally high valuation, a trapped product).
The giants are seemingly moving fast because they have comically obscene amounts of cash pouring in and can't figure out what to do with it (other than share buybacks ala Facebook, or other capital return programs; in Google's case, they're just piling it up or buying real estate). So they spray it all around. A billion dollar acquisition mistake is meaningless if you're Microsoft and generating ~$40b in operating income (next four quarters). For Snapchat, it'd be fatal.
Google's net tangible assets are up to $149 billion now. They're starting to look more like a financial company than a tech company. Like a private equity company that happens to own a lucrative search engine monopoly and a media company (YouTube). JP Morgan's net tangible assets by contrast are $182b and Bank of America is $170b.