When you look at your browser window when you click on Review Match for anyone, there will be a URL of this form and you can figure out your own ID in the system. http://dna.ancestry.com/atMemberMatch.aspx?id=45133&mid=...
45133 is my AncestryDNA user ID number and mid is the match ID number for record.
The user ID appears to track the number of customers. Off the top of my head, the highest number posted anywhere was about 107,000. 23andMe has reported 180,000 customers and FTDNA does not give out the Family Finder numbers, although it must be considerably smaller based on the number of matches people report in the two systems.
I highly recommend including your match URL when you correspond with someone. If you tell them to substitute their user ID number in the URL, they "should" be able to go straight to the match record for the two of you instead of hunting through their match list for your user name.