Many organizations (including Google on their home page) encourage people to vote in the US election without specifying a candidate or political party. Why is this done? Are they assuming that the users of the site are more likely to support their organization's political agenda than not support it thus making it a net positive for the candidate the organization supports?
I'll focus on Google, since as you noted, they were a prominent organization that was encouraging millions of people to get out and vote.
Assuming that Google does show the banner to everyone equally*, there are two possible rationales:
Google altruistically wants to encourage democracy
Since democracy only works if people vote, Google is encouraging us to do our civic duty and choose the candidates we think will best represent the country. Thus the banner is just a public service announcement, and not a way of swaying public opinion.
Google wants candidates from the Democratic Party to win
This banner is pretty much a "Get out the Vote" drive, which is something that Democrats have typically organized, but not Republicans. Shaun King from the New York Daily News described it pretty succinctly:
For decades, the "Get Out the Vote" campaigns and voter registration drives have been driven by liberals. The logic is this: the bulk of unregistered voters in the United States have Democratic leanings. The more people who are registered and the more people who vote increases the likelihood that Democratic candidates will win.
Additionally, voters who are younger and more comfortable with technology (i.e. those most likely to use Google) tend to have Democratic leanings. So if they can get their users out to vote, there will be more votes for Democratic candidates.
Note that according to OpenSecrets.org, Google has consistently donated more money to Democrats than to Republicans. In 2004, 98% of Google's political contributions were donated to Democrats. Take that as you will.
* As Philipp noted in the comments, Google does have enough data that they could selectively show the message to people they actually want to vote (e.g. if they see that you are searching for things like "Canidate X is stupid" and Google likes Candidate X, then they could choose not to show you the banner).
There are two ways you can look at it:
The rosy view would be that a larger turnount means that the result more accurately reflects the views of those eligible to vote. A low turnout, such as that caused by bad weather, results in only the most motivated voters casting their vote, which produces a biased result. Those with the strongest views are the most likely to vote, so encouraging a higher turnout tends to produce more moderate, and arguably fairer, results.
The cynical view is that a higher turnout produces a stronger illusion that the candidate has popular support. Those who support the existing system and who are generally in favor of greater government power would prefer that whoever get elected have a great turnout so they have a stronger claim to legitimacy. Low turnouts suggest that people don't trust the system.
Optimistically: There is a point of view that voting is, in and of itself, a good thing. For this reason, voting is mandatory in some countries, although questionably enforced in most democracies.
Cynically: Democrats tend to do better in high turnout elections in the United States. So someone who wants to support Democrats may very well do so by advocating that people vote. Not necessarily exclusive with the optimistic view.
Another reason is that even without showing the message to just Democrats or tailoring the message to Democrats, a neutral message on Google is more likely to reach Democrats. Democrats are on average more likely to use Google on a given day than Republicans. This is because younger voters trend Democrat.
Realistically: If they would say, "If you support Democrats, don't forget to vote!" or similar, then it actually energizes non-Democrats more than Democrats. Democrats see that message and either agree or think it's crass. The people who agree, were probably voting anyway. The people who think it's crass feel slightly turned off and may not vote. Meanwhile, non-Democrats see that and get angry. If they weren't voting before reading that, they definitely want to vote now, just to stick it to the partisan idiots who posted the message.
Chances are that all those reasons are true. They really believe that voting is good in and of itself. They want turnout to be high and Democrats to win. They realize that a neutral message is more likely to achieve that. Demographically they know that their users are more Democrat than Republican.
They may even feel a bit guilty about promoting more to Democrats than Republicans. If so, perhaps they comfort themselves with the thought that if Republicans want the same encouragement to vote, they should use Google more.
All this assumes that Google feels closer to Democrats than Republicans. And there is some evidence of that. For example, in 2010, Google was the second largest contributor to the DNC (Democratic National Committee) but didn't make the top twenty-four on the Republican National Committee (RNC) list.
In the US, voter turnout on a good day might hover around 50% -- which is pretty low. Encouraging people to vote is simply a good thing for a number of reasons.
- higher voter turnout, more representative the government will be
- encouraging people to vote = encourages people to be more politically active
- promoting civic participation is a message that most people would see as a 'good thing' (ie, good PR)
- pushing a particular candidate might appeal to half or your customers. But would likely also turn the other half off (ie, that would be bad PR)
And, yes, most data shows that the more people that vote, the likelihood of there being more votes for democratic candidates. Do large corporations benefit most from democratic policies? That's highly debatable. But certainly certain industries very well may.