What’s your social capital? PeerIndex is a new semantically-enabled service that purports to tell you.

The idea behind this service is to provide a relative measure of an individual’s online authority, and to do it by analyzing the person’s public activities in Twitter, Facebook, LinkedIn and blogs. You can elect to share or not share Facebook and LinkedIn info, but the company says that the more data you share, the better your chances are to improve your scores and index ranking. It explains that it has algorithms that take into account semantic factors (presumably to understand the nature of the content you tend to discuss, retweet, etc. – and its relevance to the dozens of topics it currently tracks as it derives scores), and social network input and sharing features (which appear to be less about follower or friend counts than it is about sharing relevant activity with the audience you want to impact). The topics it covers range from Google Inc. to cloud computing to social media and Chinese biz & economy.

Why would you want to flesh out your social capital details to rise to the top of its opinion leader ladder? There’s pure egoism, of course (assuming you do well on the rankings). But it could also make you more valuable to the folks that PeerIndex would seem to be most interested in: the marketers, advertisers, brand managers, PR pros, and the like who’d pay to know who the real authorities and opinion-makers out there on the web are. In a statement company founder Azeem Azhar said, “I’d like to think we can become the global standard of social capital. Marketing, advertising, communications and brand-led businesses will all benefit from a seismic shift in reach and penetration that PeerIndex delivers.” It can build customized topics to customer requirements.

It’s an interesting idea, and one that other start-ups have also begun exploring in their own ways, such as SmartRealm. Knowing who the opinion makers really are is the flip-side to sentiment analytics – as valuable as it is to know generally the positive and negative impressions of your product or service on the web, it is equally valuable, if not more so, to know on whom you should focus your energies in helping to correct negative perceptions or foster positive ones.

Back to what’s in it for the common man: Azhar also said in the release that, “PeerIndex will encourage the development of a mutually beneficial eco-system of high value opinion makers and relevant organizations.” As we discussed here , there could be an ROI for those who are in the king-maker spot, so to speak – though potential perils, as well.

The service expects to get more granular, too. Right now it builds scores for authority, activity and audience across 23 topics it publicly says it tracks – but it claims to actually track several dozen topics not in the publicly disclosed set, and says more are being added. Also right now, your audience score reflects a global value rather than for a specific topic, but that’s supposed to be shifting to show one’s expected audience on a given topic. Your aggregate PeerIndex score is compiled based on your overall authority, activity and audience scores. Scores are normalized, so a PeerIndex score might look low even if you are among the top authorities. As the site explains: If you are in the top 20% by authority in a topic like climate change, it means you have higher authority than 80% of other people who we measure within this topic. Your normalized authority score for this topic (the one displayed on your page) will be in the range of 55 to 65 (that is, significantly lower than 80).

Got that? For fun, we took a look at the top authorities in a few categories:

(By the way, I’d just like to report that I found myself ranked in the top 5 percent of opinion leaders for social media – not that I’m asking for luxury gifts or cash from parties in that space. Really. Story leads always welcome, though…..)

Also, have some patience with the site in its beta form — it seems to take a bit longer than you might expect to serve up its feedback.

• Don’t forget to propose your startup for our Semantic Web Impact Awards. The deadline is Sept. 15.

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