King Kill'em, overall ridership has declined relatively consistently since 2013, as can be seen on Metro's basic ridership stats page:
https://www.metro.net/news/ridership-statistics/, so the recent drop in gas prices is likely a small contributor.
I find it a bit odd that the recent stories about Metro ridership rarely mention service disruptions due to maintenance work as a contributing factor. It's simply ridiculous that for the past few years the Red Line has run only every 20 minutes on weeknights after 8:30, with practically useless Purple Line service. Not to mention the frequent mid-day and weekend maintenance-related disruptions on the light rail lines that reduce headways to 24-30 minutes. This is far from world-class to say the least.
In some more optimistic news, Streetsblog has a look at expansion projects that will be getting some planning priority at Metro:
http://la.streetsblog.org/2016/02/02...dy-initiative/