Influence arriving via social ties may be relevant for a person to decide to buy a new product, share information, or to adopt a new behaviour. However, quantifying social influence is a difficult task, even in online social systems where the interactions and communication content can be closely followed. Here we study the information susceptibility and adoption thresholds of users on Twitter. We consider hashtag and retweet adoptions on different aggregation levels: items, users, and topic groups, and study these adoption mechanisms characterized by diversified or repeated influence stimuli. We find both metrics to be heterogeneously distributed, correlated, and dependent on the topics and aggregation level of social influence. We show that users adopt retweets easier than hashtags, and find that new influencing neighbors can effectively trigger adoptions. Our results may inform better models of adoption processes leading to a deeper empirical understanding of simple and complex contagion.