Poetry.com is a SCAM directly related to the National and International Libraries of Poetry, as well as Nobel House Publishers and the Poetry Society of America and with them has been the subject of numerous exposes by ALL the major news networks
For specific information on these and other SCAMS, check out
http://windpub.com/literary.scams/endall.htm
All the above have been subject to exposes by the major news networks and many articles on the Internet and they are designed entirely to take your money and offer nothing substantive in return.
Take care, avoid the "free" contests, and confine your submissions to legitimate markets which can be found in the Writer's Market Poetry Yearbook, or at
http://www.writerswrite.com/poetry/markets.htm or
http://www.drowningman.net/poetrylinks.htm among other places.
As far as your Poem goes, I agree with what has been written before by Hapi Sofi in saying that they don't own your work, (whoever THEY might be), unless you have SPECIFICALLY signed a CONTRACT granting them ALL rights, those right remain yours, and NO Organization" can claim the rights to your Poetry simply on the basis of submission.
I understand your original post was in regard to a Greeting Card company, and I can't understand how any legitimate operation would make such a claim except to intimidate you for fear of inadvertantly publishing something similar in the future, and suing them for infringement, and any such conflict would hinge on proof of authorship (as evidenced by Formal Copyright) and proof that they had the opportunity to steal the work, (which basically means you should maintain proof of any submission or any other contact with them).
The problem with Poetry.com and their bretheren is more complicated, as any work published by them will be CONTAMINATED by their REPUTATION regardless of any rights signed away.
Bottom line is that unless you've SIGNED your rights away, noone you have submitted to can make any claim to your poem.
JRH