The Mysterious 50500

Last Night I was working on a job site, and I had checked into it using
Foursquare (twice, the first time a typo).  I was wondering if there was a way to undo a check-in,
or something similar.  So, of course, I texted “help” to 50500 (the US Shortcode for Foursquare)
and I got back the following message:

2 Vicinite alerts/wk,
tones/video clips: $0.49-$9.99/mth+ msg&data rates may aply.  Visit
vicinite.com/index2.html or 8666443345. STOP 2quit.

Wait, what? 
That has nothing to do with Foursquare at all.  What is going on here?  So, I decided to
google the shortcode for more information, and found out its ALSO used for Contxts, another service I used. 
Just to make sure, I texted “David” to 50500 and what do I know?  His business card was texted
right back to me, with the sender labeled as “Foursquare” due to me having set the number as Foursquare’s in
my address book.

This is kind of fishy, in my opinion.  How,
and Why do these three services share the same
shortcode number?  They seem to have nothing to do with each other, either.

So
then… how do they pull it off?