Yes, it concentrate/centralize power. If you want a new aristocracy, continue buybacks until a share is worth so much, people will start to buy share fractions, with all the shit and overhead that'll bring.
How is that better? With buybacks if I want to reinvest I don't have to pay taxes on them and if I want to realize the gains I can decide when to do that myself (of course seem more like an issue in Europe and with companies which only payout yearly).
Because it encourages longer-term ownership, and gives shareholders a stake in the future cash flows of the business.
Like I said, it's the tax treatment that basically stopped growth companies giving dividends. I think that we should change said tax treatment and also make buybacks illegal but that's a bit more controversial.
In any case, for both G and FB, they'd need to do buybacks anyway because of their employee share programs.
Buybacks have the same effect of encouraging long-term ownership.
Given a certain amount of money, spending it on stock buybacks vs spending it on dividends returns the same amount of money to each HOLDING shareholder.
In one case, they receive a small sum. In the other case, the value of their stock goes up.