Here are a couple of MLS basics that might help. I realize that sometimes it's all Greek...to me too!
1. The listing is actually with the BROKER, the Realtor with whom you signed the listing agreement is an agent for the broker...and an agent for you.
2. Brokers have the ultimate control of the listing. Brokers are members of associations, and those associations have multiple listing services. The broker decides what other online site are fed the listings.
One of the issues that is this: If a broker uploads his listings to Realtor.com, realtor.com then sells leads (inquiries about the listings) to third parties (such as servicemagic.com). Servicemagic.com sells the leads to the realtors for $300 for ten leads. So the realtors in a sense buying their own leads.
Some brokers and Realtors object to this, so some brokers and their associations are being selective about where to upload their listings.
Example. Keller Williams, my company, opted to have our own MLS. Our listings are uploaded to Trulai automatically, and Googlebase. If a broker does not want their listings uploaded to a particular online aggregator of listings, it won't be there.
Talk with your Realtor. There is a learning curve to all this. Just remember that 90% of the time, the buyer that purchases your home was brought by an agent...so marketing to agents, although not as noticeable to the public, is very important to selling homes.