The #1 measure of sucess for an exchange standard is how many transactions per second take place via that standard. This is of course a pain in the neck for an emerging standard which since for a long period that number is zero.
Advocates of a standard will substitute other metrics to both create the impression of success, but also to measure the precursors of sucess. They might report how many users have could transact if they wanted to, i.e. how many have the ablity to send/recieve a fax. Or they might report that certain large vendors have indicated they will adopt, as Microsoft does or doesn’t intent to support SVG. But, these measures aren’t the #1 measure and until you have the #1 measure the standard’s success remains a risky proposition.
Here’s a nice example of something picking up real momenteum by that measure.
“a brand new weblog is created every 11 seconds. We’re also seeing about 100,000 weblogs update every day as well, which means that on average, a weblog is updated every 0.86 seconds.”