I think the scenario of coming up to a table where I am having dinner with a friend is a gray area. I think it depends on the approach. If you are going to do this, make sure and say hello to both parties at the table and make your intrusion brief. Write your number on a napkin and just drop it off, or ask her to come speak with you before she leaves, or just send both parties a drink. But I would find it rude if you just came and plopped down at our table.

I personally don't go places with the intention of finding single people. I find events and places that I am interested in and find single people there. It at least gives me the start of something in common.