I just can't see either of these sites remaining relevant when fb takes over the market. I use both offerup and letgo, and each one works well in some geographies but not others. For example, lots of people post things to offerup in the DC area, but not to letgo. At the same time, I find lots of stuff on letgo in the Pennsylvania area. FB is going to use their network to overtake all geographies.
On that topic, I think fb tried to enter a number of market but wasn't always successful (looking around there's things like coupons / gifts / credits).