I always looked forward to the salsa birthday dance. I still send wishes to my friend's for a good birthday dance.
We followed an unspoken etiquette that the experienced salseros and salsera led off the birthday dance for the honoree, and the rest of us mingled the crowd and gently encouraged others to dance with them as well.
When someone left the honoree for a few seconds too long with no substitute partner (usually a non-regular), another experienced regular would step in and dance with the honoree while we continued to try to find the next victim... I mean dancer... to step in... and the cycle repeated until the song completed. Fortunately, it was only a 3-4 min song.
Tango seems a lot more exclusive so some of these perspectives on the birthday vals seem very different, but still interesting to learn about.