1. Yes, it means 50%. This is reported speech. "%" can be dropped sometimes in speech if it is understood that it is meant. I wouldn't drop it in writing.
2. Migration is a massive movement. Like birds migrate from north to south for the winter. It is speaking of people collectively moving from one thing (yellow pages) to another (internet searches).
3. Yes, it's a tad redundant.
4. "Listing" is the jargon used with Yellow Pages. "Category" is a suitable synonym.
Student or Learner