Something else that could be mentioned in the linked article: if someone you were with got in a cab in 2013, and they told you where they were going, and you remember the approximate time and location, you can tell whether it was their true destination regardless of how many other people were being picked up at the time, because you don't have to find the exact ride they took, you only have to see whether any rides went to the place they told you.
This search is even extremely resistant to the differential privacy suggested by the post's authors. I'd be much happier simply stating that location data is not de-identifiable, and no-one should use a cab in a city that logs location data if they aren't happy with an adversary knowing where they went.
That is, are there locations A and B such that there are matching trips to locations M1,M2,... at times T1,T2,... i.e. (A,M1,T1),(B,M1,T1),(A,M2,T2),(B,M2,T2) and perhaps reverse trips (M1,A,T1+x) etc?
Further classification of M* -- hotels, for example -- could classify the nature of the meetings. You might be able to identify the addresses of people having affairs, or other deliberately secret rendezvous.
I was concerned when this first hit HN because I have a friend that lives in a fairly sparsely part of town and his (now ex) girl friend has a possessive ex-husband that doesn't like her seeing other men. He isn't going to be able to make sense of the data himself, but if someone weaponizes it the way you are talking about it could be a real problem for people with stalkers/psycho-exes.
 Specifically things like where notable strip clubs are
As an attractive girl, you do not want to be walking / taking public transport in certain areas of town at 6am, it's a sad reality.
Please show this as an example to people that say "Why should I care? I have nothing to hide".
~85M rides with 0% tip is "interesting".