It's a lighthearted question, often meant to be humorous and sometimes a bit gossipy. The person answering might reveal their crush, or they might play coy and give a funny or evasive answer.
Here are some possible interpretations of "You fancy love" in a slam book:
* Who do you have a crush on?
* Who do you think is cute?
* Who do you think is attractive?
* Who are you secretly crushing on?
The exact meaning might vary depending on the context and the tone of the slam book. It's ultimately a fun way to spark some gossip and maybe learn a little about someone's secret affections!