I'm an ancient historian. We don't have a lot of evidence that survives (and we don't go archive diving like so many of my peers). However, we don't just study diaries or daily logs -- historians study primary sources both material and documentary. Historical accounts are not created by looking at one great source and going, "Man, Jim was really on point from 2006-2008. Way to go, Jim!" It's by combining Jim, Mary, Suzy, Juan, and Max, even if Max and Jim disagree with each other completely.

So what will future historians use? Music counts, maps count, art counts, obviously books and diaries count. All of these primary sources will help historians study the time period under their interest. Facebook, tumblr, instagram, and twitter will quite obviously be primary source material in the future -- provided that it is archived and logged in a way that is accessible and understandable to future scholars. The archiving of the internet is a huge issue for archivists and historians, since things can end on the internet at not exist on the Way Back Machine -- how would you do a history of Geocities today? In 20 years? The problem, then, is not the depth of the source material, but the manner in which it is collected and maintained.

More seems likely given the nature of the Internet and the care we take in the modern age to preserve history. And things like Facebook and Twitter serve the same purpose as a journal, I don't know what you mean by deep and meaningful, but there is plenty of coverage of world events and people reacting to those events and historical journals aren't always deep and meaningful.