Last week, a top Twitter executive said that the company would soon not only be indexing tweets in its search results but the pages they link to as well. The new service hasn’t even launched yet, but already it’s got plenty of competition.

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At least two startups have now introduced their own Tweet-plus-Tweet links search engines. On Tuesday, OneRiot started searching pages people link to on Twitter and those recommended on Digg, too. A second site, Tweetmeme, which had already been tracking the most retweeted posts on Twitter, has also expanded into searching linked pages, including text, videos, and images.

OneRiot and Tweetmeme’s real-time search engines don’t quite fully work yet. For instance, on OneRiot, none of the results for “Intel” showed that Intel (NSDQ: INTC) had been levied with a $1.45 fine from the European Union this morning. Tweetmeme’s results did—but not-so-newsworthy articles such as a press release announcing that Intel Chairman Craig Barrett would address a conference in Arizona was interspersed with the results. The two services nevertheless reemphasize that real-time search is all the rage right now—and also raise some questions about Twitter’s forthcoming service. Sure, Twitter has the huge advantage of being the place where people are actually linking from. But beyond that, how will it differentiate itself from competitors who are already well on their way to adequately sifting through the pages Tweeters link to?