I know of a couple of professional photographers who are Instagram users, and I'm sure these guys knew full well the risks involved in using such a service to feature their photographs online, long before the big flap came down from Facebook two days ago. What these, and possibly thousands of other professionals are doing is using Instagram to market their "creativity" to the world, and not their actual photographs. To me, this is a smart move. The photographs they use in doing things this way are not their bread and butter. They are postage stamp sized "Instagrams" which happen to demonstrate their unique style, artistic flair, and sense of fun.
On the other hand, if you're a self-styled pro who is using Instagram to somehow market your bread and butter work - (and I'm not even sure I know how you would even be able to do such a thing) - then you stand to lose, not only through Instagram, but through any popular online photo sharing service, including Flickr.
I'm not one to give advice to pro photographers, as my approach is a lot more copyleft * than it is copyright, but there are basic precautions that everyone who takes the art seriously needs to be aware of:
- Know your intentions regarding your work
- Find a good way to showcase your creativity without actually giving away your work to big conglomerates like Facebook and Flickr who can (and will) actually steal it from you
- Make sure what you're sharing is downsized enough to render it useless
- Use Watermarks as an option to protect your work online
- Make sure you have your best work made into hardcopy (prints or negatives) and stored in a safe place
* My definition of Copyleft - "if anyone wants to use one of my rather shitty photographs for any reason, even though I use the public copyright license, by all means fill your boots, but I would appreciate your writing me a quick email to ask my permission - it will almost certainly be granted, unless you are a sex offender".