Earlier quoted context omitted.
How much information on sushi places in Palo Alto is your grandma pumping into the system to dominate that sector? Is she checking into multiple sushi places dozens of times a day? It's like supplementing every google query with -site:4chan.org on the off-chance a search for a camera lens spec or node.js tutorial would lead to 4chan.
Replace "grandma" with "person whose taste in sushi is very different" if it makes you feel better.
1) Rank by popularity among all friends.
2) Rank by global number of check-ins.
3) Rank by a one-off friend's preferences.
You think (3) is the way something called GraphSearch works?