That's a very good point too which I missed. People don't care about whether their friends are using Uber or some other app. They care about whether the ride is cheaper. It's a bottom of the barrel pricing game in ride sharing apps.
They also care about wait times. And drivers care about utilization.
"Taxi apps" definitely have network effects from the two sided market, though not as strong as a social network.
From what I have found personally, the wait times on Uber and Lyft are the exact same in my area in Canada. Every driver which is on Uber is also on Lyft. So I don't think the network effect is helping them much as riders aren't really getting anything different in terms of wait times. Also this is even after Lyft barely being about 1 year old in my city while Uber has been around for many years.
Uber pool gives it a very strong network effect. You need enough riders going in the same direction as you at the same time. When I do my morning commute, I can't take Lyft because there isn't a critical mass and there isn't room for two ride sharing networks. Uber is consistently cheaper and has shorter wait times.
The main difference seems to me that Facebook enjoys a network effect: you can move to a different social network, but who will you find there? While you can download as many taxi service apps as you want and use one or the other, the one that has the cheapest offer. If an Uber competitor comes up with autonomous driving first and offers ride for half of Uber's price, people will switch instantly.