I conjecture specifically that presenceless messaging systems are more dangerous this way, both because of random reward and because of social risks surrounding attention allocation (engendering the default expectation of always-on). If you don't know when the other party is online, you send messages whenever, and sometimes you get rewarded with immediate feedback. Conversely, if you don't know when they're going to message you, then when they do, you might feel like responding right away is “supposed to be” no big deal for you even if it turns out to crowd out other things. IM where people are visibly only signed in when they're probably ready to chat, or telephone calls where both parties are expected to pay attention or hang up, allow conversations to have more explicit closure that releases attention for other activities.