[niftich]

My speculation is that Yandex wishes to gain a foothold in the US/Europe/ANZ market for display ads; they already command a significant share in cyrillic countries and Turkey; Turkey is incidentally a large Twitter market.

The primary cyrillic social network is vKontakte whose popularity in those regions is to the detriment of Facebook. Meanwhile Google is a serious threat to Yandex' search share (and therefore, ad money), so expanding beyond their home base would be a good hedge against Google.

If you look at Yandex's portfolio of services you'll notice they're missing a general-purpose social network, and their coverage looks similar to that of Yahoo, with elements of content production (like Verizon/AOL/Yahoo) sprinkled in. But unlike Yahoo which was cash-strapped and couldn't figure out how to monetize Tumblr, Yandex has cash on hand and Twitter would be an expensive, but very strategic purchase.