I neither use Facebook nor watch television, so I'll skip 1 and 2.
Number three is a very interesting question. Why did Reddit and Digg descend into their present state?
I think something other than "the userbase has grown" is going on here. I think one major reason is that you see others' opinions before you decide your own. As a general rule, any Internet poll prompts you to vote before showing you the running results, to attempt to preserve independence of opinion. Reddit and Digg, however, breach this.
Secondly, in both Reddit and Digg, any controversial item leaves public sight; only those items that most approve of reach and stay in the front. Those items only of great appeal to a subsection of the user base either fail to reach and stay in the top, and, as a result, simpler items with more general appeal begin to dominate the front.For Reddit; this was the rise of political articles; for Digg, which I am less familiar with, I'd imagine it involved less technically in-depth items.
The above system accelerates; as content begins to represent the cross-section of the interests of a larger, more diverse group, the site attracts an even larger, even more-diverse group.
The squashing of controversy and propagation of opinion combine so that commenting maintains a status quo: The posts that are upmodded are known to be upmodded, and, thus newcomers will associate these types of posts with being worthy of upmods and will upmod them themselves. Due to the desire for social status, those newcomers bold enough to begin making comments of their own attempt to imitate the style accepted by the community as "upmod-worthy." And thus, newcomers learn to achieve a style that causes their posts to be upmodded.
The above two points combine: The oldtimers tolerated newcomers; the newcomers tolerated newer-comers; due to the logistical nature of the system, each "generation" outnumbers the previous. The newer-comers whom the oldtimers indirectly "let in" may create comments repulsive to many of the oldtimers that get upmodded to the top; this slowly disillusions the relevant oldtimers, causing them to leave, solidifying the community's shift.
And there, that's Digg, according to my understanding. I must admit I've used Digg little and do not have a firm grasp on how its system works, so everything I say about Digg should be taken with a grain of salt; on Reddit, I am more confident in my ability to speak truthfully.
Reddit's story goes further: In Reddit, a controversial item where one side has an advantage can reach the front page. Thus, assuming it involves the general cross section of readers, the ideology that originally had an upper hand will get its content to the front and the opposing ideologues, primarily reading items they disagree with, leave. The former ideology solidifies its hold; future opponents of said ideology get discouraged from joining; groupthink evolves.
And thus, my theory for the state of Reddit today. Reddit's original techie audience created the status quo of commenting style. They drew in their friends and peers from various communities via word of mouth. The new users tolerated and contributed to the technical articles that were reddit's genesis; the techies tolerated and contributed to the nontechnical articles being brought in. The cycle occurred again, and, though the technical bias in the community was preserved through visible, active posters, a shift toward articles with broader appeal was well underway. Since the original techie user base likely contained more liberals and libertarians than conservatives, the former two ideologies, both of which involve laypeople and can be associated with most topics, began to heavily get their content into the front. Liberalism and libertarianism tolerate each other somewhat, but liberalism was stronger due to its numerical advantage among the general populace. The hold of liberalism expanded as outlined.
As one might expect from the above paragraph, the front page of the Reddit of today has articles on healthcare, global warming, and other politicized material; some items supporting Ron Paul; and a whole bunch of items with broad but shallow appeal, such as pictures of cats.