Every FD site will usually have APRS on line. Remember, you can contact any other FD site in the world on your APRS channel. (144.39 in North America).
Just send them an APRS text message.
Of course, this assumes you know their callsign. To facilitate learning who else is on the air, send a CQ FD message to the APRS CQSERVER and your CQ will go to every other CQ FD site on the planet. This has nothing to do with FD rules and points. It is just a way to have fun and be able to contact any FD site without knowing apriori their calling freq.
Just send an APRS message to CQSRVR and make the first two words of the message be CQ FD .... for example, "CQ FD from Bob in Annapolis, MD". These do not count for points, but are a great way to have ham radio fun with other sites.
This will log you onto the CQ FD message group and from then on, you will get ALL OTHER such messages from all other such stations. When you DO see an incoming CQ FD message, now then you know their callsign and can MESSAGE THEM DIRECTLY from then on.
To keep the load down, you can only send one CQ FD message once every 30 minutes, but you will remain logged on for up to 12 hours. After no activity, you will be dropped.
Oh, for more info about APRS and Field Day, see http://aprs.org/cqsrvr.html
Bob, WB4APR