identifying those who'll spread your message
As a percentage of your overall community it's difficult to say how many you will have, but you will have them (to varying degrees). Initially they will probably be staff or family members, but if you keep a community active and engaged you'll soon start to see more - from experience I'd say it's probably around 1 in 300. Without going to the trouble of paying for software to identify these people for you (you'll want to be considering this when a social channel goes over around 5000ish, as that seems to be the 'confusion tipping point' (for me anyway)) just create a decentralized (Google Docs) spreadsheet of any individual comments, the name of the person doing the commenting, and the name of the forum, blog, review site (like Yelp or Review Centre), or social media channels (Tumblr, Twitter, Facebook, YouTube, etc.) where the comment appears. Comments tend to stand out better then simple button clicks ('likes', yada, yada) or shares, and commentary is a stronger mark of evangelism. If you find an individual (or firm) chatting about you across multiple channels they're definitely what you're looking for, but people can be passionate in just one channel too (especially social) so that's not a dominating indicator. What they are saying, and the frequency they are saying it, are usually the big clue.
NB: Just a quick note, a Brand Evangelist is not necessarily an Influencer. Influencers are the peeps with the big networks and followings, but they may not give a pair of fettered dingos kidneys about you or your brand. If you can get a Brand Evangelist who IS an Influencer then that's great, but for this exercise they're two different things - as a casual observer I like to use the Klout tracker Chrome extension for Twitter to keep an eye on Influencers who might be passing stuff to their followers, and I run down my list of recent engagements in the 'connect tab' to see if anyone stands out. I'm not a massive fan of the likes of Klout, Trackr, or Peer Index as (if the truth be known) I don't honestly trust the accuracy of their figures - but they are a handy first indicator.
Fostering NEW Evangelists
creating people who'll sing your praises
To make new evangelists in social media you need to find new opportunities to make conversations. This means going out there and talking, giving things away, soliciting reviews, all the usual stuff. Reach out and partner up with suppliers, people who share similar ethics (like green recycling policies, or those who are involved in the same charity work as you, etc.). Be the 'real deal' and join conversations about things you or your brand are interesting in - don't just flog your widgets. You need to help people, talk to people, and give them practical solutions. Try using social to run simple competitions, even just for small fun prizes - basically, stimulate people to talk about you.
The obvious method is to just push the boat out a bit and to give good service - for example, my break light went out and I was passing the dealership I bought my car from so I popped in for a bulb. They insisted on fitting it for me, and not charging me for the bulb. I'll be an evangelist for Smart of Derby now forever. It cost them 10 minutes, a bulb, and a smile, and now they have my good will across my entire network of friends, fans, and followers.
Tools You Can Use
If communities get too unwieldy for manual comment collation, or you want to examine trends by individuals in 'likes', 'shares' etc., you're going to have cough up some pennies. Tools can be well worth it as part of a short-term out reach campaign, or for larger communities over longer periods. There's plenty out there for Influencers, but not that many for Brand Evangelists (i.e. filtering out the Kool Aid drinkers in specific communities).
ViralHeat is pretty simple, probably the best for this, and has a 14 day free trial then only $9.99 a month. There's also Twesier, which has has a Pro package that let's you sort your friends and followers by popularity, influence, activity levels, etc. with 7 days free trial and then £9.99 a month.
What To Do With Evangelists
Once you know who these people are it's time to get creative. How about inviting them to events or to come and take a look behind the scenes? How about sending them new products or asking them about new services? How about just sending them a bottle of wine when they've just a special occasion, and saying congratulations? How about giving them some money off codes to spread around amongst their friends. There's lots of options, and they all foster sign-up, inbound links, good will, word-of-mouth, and eventually sales. It's really down to what suits the brand personality and to what suits your wallet.
Hope that's of use, and if you know any more tools for specifically identifying a Brand Evangelist (over influencers) please let me know, I'd love to add them.