HelloTalk
Best for: Learners who want a social community as much as conversation practice.
HelloTalk is the closest direct alternative to Tandem and arguably the most popular language exchange app overall. Its strength is the community: the Moments wall, the audio rooms, the corrections feature. If you enjoyed Tandem's social side but want a bigger user base, HelloTalk is where to go.
The catch: HelloTalk has the same fundamental problems as Tandem. Most activity is text-based, not voice. Users can be unreliable. Some of the platform is used for flirting more than language exchange. If those were your reasons for leaving Tandem, HelloTalk won't fix them.
Cost: Free. HelloTalk VIP is around $8/month for extra features.