In my experience, it depends on where my Sims' homes are located. The closer to town center, the more visitors (I only have Vanilla, so I can only comment on that). I've found my Sims get more drop-bys when they purchase pre-made homes. But even with custom homes, even far away from town, certain folks always seem to drop by. Victoria Andrews is one. Perhaps it's because she's a journalist. Perhaps she feels there's a juicy story to be had!
Emma Hatch and Nick Alto like to drop by, too. Not at the same time, though.