This is actually the reason why I'm downloading it right now and giving it a chance. I don't have a (personal) Facebook account and I don't ever plan on having one. That effectively lowers the barrier of entry for me significantly.
Didn't know that. Interesting. So are you generally sharing with friends or strangers? I've never had anyone ask me to join instageam to see a picture, which is why I ask.
You post them in your stream, you can choose to only show your photos to people who you've friended or you can choose to post them all publicly. There is no middle ground.
You can share the photos outside of instagram on Twitter, Facebook, Posterous and via mail. People can view the images in their browser if you send them a URL or if they use a third-party website to browse photos.
It's quite flexible and a lot of fun sharing and commenting on other's photos. If you want to get more activity on your photos, you need to #tag them. Some people go overboard with obnoxious 20-30 tag posts.
But you can link it to FB and get notified when friends join.. which I must say has likely been a large driver of their growth. How else would you know which friends are already on the service?