Ask HN: When is it okay to email the users?

So a problem I'm having is that I had a big wave of users who signed up for my website (http://thingist.com), used it for a while, then disappeared...

A few people have told me "Well, send out an email to people who haevn't posted for a while. Or send out an email to people who registered but never posted at all."

This seems like a decent idea, but I'm concerned about being "spammy". Is that concern valid?

Are there any best practices guides for this? Or advice you can give?