This came in over an e-mail list I’m on dedicated to MediaWiki (the software behind Wikipedia). It’s a breakdown of where the traffic comes from for all Wikipedia-related sites:
The answer is Google, mostly. No surprises here. In fact, I thought the Google numbers were low until I remembered there that this table doesn’t exclude internal traffic from other Wikipedia-related pages (rows #1, #5, #6). Note that no non-search sites made the short list. As much as Wikipedia is linked to by other sites, the traffic from these links are negligible (something less than 1.6%) compared to the traffic arriving from searches.