How to determine the weights of features when calculating the weighted average?
Retweet and Favourite are totally different features and although they are correlated with the popularity of the tweet, the exact correlation will be subjective to the training dataset you are using. 

One way to come up with the weights is: Information Gain 

This paper tries to come up with a weighted average using the user and tweet features for the prediction of retweet counts for a given tweet: http://www.nlpr.ia.ac.cn/2012papers/gjhy/gh154.pdf

Steps by step procedure:
1) Give every tweet a popularity score between 0 to 5 (You have to label your training data) 
2) Calculate information gain for different weights of Retweet and Favourite count (refer: https://homes.cs.washington.edu/~shapiro/EE596/notes/InfoGain.pdf)