Social media influence is a bit like oxygen – we "know" it's there and we know in some sense that it is essential for “life” if we’re in marketing, advertising or public relations, but can we capture it in a jar and observe it? No. Well, at least not entirely.

Any number of tools – freely available on the Web – purport to tell us our overall social media influence or at least a slice of it (a la Twitter).

While none is perfect or complete yet, using a combination of them can prove useful.

Here then is a roundup of tools that measure influence in the social web:

Broad social web tools

Klout: Klout currently track a user’s Twitter, Facebook, Linkedin and Foursquare activity. It is reportedly looking at Google+ integration. From the Klout blog: "We think you are influential. Klout isn’t about figuring out who is on the 'A-list.' We believe that every person who creates content has influence. Our mission is to help every individual understand and leverage their influence."

PeerIndex: How PeerIndex describes what it measures: "PeerIndex: a measure of your online social capital. Topic fingerprint: a snapshot of what you talk about. Topic resonance: how much other people find what you share valuable. Comparisons: compare yourself to your friends and peers. "

Twentyfeet: What Twentyfeet says it offers: "Aggregated stats in one place. Your overview of Twitter, Facebook, YouTube, bit.ly, Google Analytics, Myspace and more. See how your key performance indicators develop over time. We nudge you when your metrics change significantly."

Empire Avenue: From the home page: "Invest your social capital in people and brands for free using virtual currency" and "Engage and expand your social networks, and learn how to use social media more effectively." Whether a virtual stock exchange and virtual currency truly relates to social standing is still hotly debated on the web.

WhoSay: This invite-only service says this about itself: "WhoSay is a service that helps artists, athletes and iconic personalities connect with their fans. When you see someone posting via WhoSay, you'll know that it's real, authentic messages, photos and videos coming from your favorite people…. As a fan, you may arrive at your favorite artist's, athlete's or personality's WhoSay page by visiting their existing social media sites." In other words, just being here means you likely have influence.

PostRank : This service says it "tracks where and how users engage, and what they pay attention to — in real-time." PostRank says it measures user activity, "the most accurate indicator of the relevance and influence of a site, story, or author." By using Postrank, in a sense, you’re advertising your social popularity so brands can connect with you.

Influencer Exchange: Appinions: From the website: "For any user defined topic, brand or issue, the Influencer Exchange helps you discover, identify, engage and monitor the leading influencers. Leveraging the power of opinions, the Influencer Exchange embraces the Web, social media, forums and news articles to provide a comprehensive view of the influencer landscape."

Twitter tools

Tweetlevel: From the How To Use page: "TweetLevel is a purpose built tool for PR and marketing to help ensure brands use Twitter effectively." From the About page: "This tool will be in permanent beta as we seek to continually improve its functionality based upon your feedback. Even though we believe that it goes a great way to understand and quantify the varying importance of different people's usage of Twitter, by no means whatsoever do we believe we have fully solved the 'influence' problem.”

Twitalyzer: Twitalyzer looks at who is in your social network and, mostly for a variety of fees will give you data across at more than 25 metrics. From the site’s Benchmarks page: "Twitalyzer's Benchmark report allows you to generate ranked lists of Twitter users based on their stated location and the tags that have been applied to their profile."

TweetGrader (formerly TwitterGrader): A site that allows users to see how they compare to hundreds of thousands of other Twitter users, see how they rank in their geographic area, track Follower history and a Quick Follower Check to see if another Twitter user is following you.

Twitaholic: "How's this work? Our Twit-tastic robots scan Twitter a few times a day to determine who’s the biggest twit." Enter your Twitter account name to get your ranking on Twitter (by followers) and by location. This site also encourages you to click through to Twitter Counter.

Twitter Counter: From the site: "Twitter Counter provides statistics of Twitter usage and tracks over 14 million users and counting." Once you’ve entered your Twitter name you can compare your states with two other users.