If it were a simple matter of flipping some switches to keep from being classified as spam, spammers wouldn't have any trouble whatsoever getting through spam filters.
The biggest things you can do...
Make sure you are sending actual email. No fake headers, no CC's to dozens of people unrelated to the person you're sending to, etc.
Keep the message simple. No fancy formatting or turning out web pages as email. Anything that smacks of advertising will up your spam scores in most spam filters, and the more prettified the text the more likely it'll trip filter scores.
Configure SPF and make sure your mail server isn't obscured behind multiple routes or anything that can make it look dodgy when being reverse look-upped.
Have instructions for your users so they know what to look for; if they have their own filters this will help with white-listing.
Have an opt-in list so you're not sending emails to these users unless they want it.
Does your social site have its own built-in messaging? It might make sense to encourage people to use the in-system messaging rather than email. Email should at most be an adjunct to the goal of bringing people into the site and using the site.
There's nothing you can really do to prevent being classified for spam except make the messages legit and don't tick off your users; someone may forward it to the wrong places and you'll end up in an RBL. Only send messages to people who want it. Take a note from Facebook's page...their notifications that I opted into are a simple "hello! Here's a message from XYZ, click the link to view the thread!" I don't think there's anything fancy or hidden in the message (but then again I'm also known for forcing simple text and non-rendering html, etc. in my email to reduce the chances of getting more spam through or malicious links. I could just be blocking out extraneous crap. But regardless, my spam filters haven't blocked any notifications from them.)