The latest search market share numbers are in from Hitwise: in the four weeks ended March 31st, Google (GOOG) racked up fully 64.13% of all US searches. That’s up more than 10% since March 2006, and, if trends hold, Google’s share will pass the two thirds mark by August. In the same period, Yahoo (YHOO), Microsoft (MSFT), and Ask (IACI) all lost share. As the old Wall Street hands like to say: “Liquidity begets liquidity.”

Clearly, this is not good news for the search also rans, particularly as it covers a period when all of them made major and costly improvements to their search engines. Ask.com for example, rolled out an impressive local search service in 2006 and also greatly improved its image searching, and yet its share declined nearly a half a point, to a fragile 3.48%, in the course of the year.

Conventional wisdom has long held that Google is vulnerable to vertical attacks, i.e. search engines that carve out category niches. The new Hitwise data suggests, however, that Google is actually gaining influence in valuable verticals. Search engines overall grew in influence over the year, accounting for more of the total traffic sent to sites in the categories of travel, health, shopping, business, and entertainment, among others. However, in all of those categories, the growth of Google’s share of this “upstream traffic” flow outpaced that of its rivals. In shopping, for example, Google’s share grew 14% while, taken together, all search engines increased their share in that arena by only 6%.

Of course, web market share numbers are notoriously fallible, and these will require further vetting. Meantime, those desperate for any sign of Google weakness, may take some consolation in this recent PC Magazine critique of Gmail. Delighted GOOG investors wishing to further fuel their warm and fuzzy feelings, on the other hand, should proceed directly to Alex Iskold’s two-by-two matrix-laden proof that Google is, in fact, the “ultimate money making machine.”