Voter turnout has a liberal bias
The higher the turnout, the better for democrats it is, because those who register republican are more likely to actually vote than those who register democrat - probably because republicans tend to be older and older people are more likely to vote. Thus, there is a correlation in elections where the higher the overall turnout the better the democrats do, at least on a national level.
So if Facebook merely presents the "I voted" button to every one of its users in the USA, without trying to slant it by party, age, or any other factor, I'm sure in certain quarters it would be claimed they were trying to influence the election.
Though I wonder if that would be true for this election, which is unique in that both candidates have record high unfavorability ratings, so most people casting a ballot for Trump are doing so because they hate Hillary, and vice versa. Greater turnout might induce more reluctant republicans to vote against Hillary than the reverse.