I don't think it would really be all that big of a deal if someone PM'ed you their number, however, I don't think it's a good idea to publically post your number. I haven't read the T/C's of S.A., however, I'm sure there's something in there that says you can't just give out your number to other people. The numbers are all unique 16 digit numbers. If you read through the first few pages of this post, you'll see what happens when too much information became public (i.e. how by posting how to decode & guess Amtrak ticket numbers, it spread to another site & then the benefit of getting free S.A. cards was shut down).

I wouldn't give out my number to someone I didn't know personally, much less post it on the largest travel website in the world, or even in a thread that's already had 13K+ views - just not too good of an idea, IMHO.