picking random user from online searching users and making a match using firebase - Android Studio

which uses the ideas of these apps:

1) LivU: Meet new people & Video chat with strangers

2) Live Chat - Meet new people via free video chat

These apps lets the users to search and connect to a random online user, i have a made a FirebaseDatabase ref("searchingUsers") for the algorithm,

My searching child screenshot

When userA and userB are searching userB gets connected to userA, but the problem is that at the time of connection userC often get connected with userB. please help me out with this, It will be very kind if any one explain the whole algo. thanks in advance!