We investigate how the spreading of messages in social networks is impacted by user selectivity for messages based on their content, competition for user's attention between different messages and message content adaptivity through user-introduced changes. An agent-based model of message spreading featuring these mechanisms is introduced and explored to asses the impact of each on statistical properties of resulting message cascades, in particular cascade size and mean length of the messages. We observe that selectivity changes the popularity distribution and is responsible for preference towards short or long messages -- long messages for low and short messages for high selectivity. The competition drastically limits message spread if selectivity is low, but has only small impact for high selectivity. Message content adaptivity has small effect of increasing popularity of already popular messages, but only for medium levels of selectivity. We explore the spread process on random and scale-free synthetic networks and conclude that the existence of hubs in the network seems to have a marginal impact on cascade size distribution. We relate the distribution of mutated message variant popularity to observations in real social media.