Most brokerages already have this setup with their listing syndication. Most MLS's syndicate your listing when you upload it to the MLS. It automatically distributes it sites like Zillow.com, Trulia.com, Realtor.com, Homes.com, HomeFinder.com, Postlets.com and many more.
If you're not comfortable putting such a large number of sites you can change the number to what is correct for your MLS.
If you're still unsure then you can remove that bullet point from the letter.