For this blog post, I'm going to focus completely on Twitter, giving only a few basic thoughts to remember when deciding when to post.
Tweets come in at a rapid pace. According to Internet Live Stats, there are about 6,000 Tweets sent every second. So, how often you should Tweet really depends on how many people are following you. If you have anywhere from 50 to 1,000 followers Tweeting five to eight times a day is acceptable. However, if you have anywhere from 1,000 to 10,000 followers, you will want to increase that number to 10 to 24 a day, possibly to every half hour.
I do believe it's possible to Tweet too often, especially if the people following you are not following many others. In that case, their timelines could be filled with your Tweets. The safe assumption, is most Twitter users have about 200 followers (according to a study by Beevolve) and they are probably following about 100 users. If you're going to be active on Twitter - the more Tweeting, the better.
My recommendation for the best Tweeting practice: if you are just getting started, never Tweet more than once an hour but never less than every three hours. If you are established with more than 1,000 followers, tweet at least once an hour, but never more than every half hour. (Keep in mind that the more active you are, the more eyes will see your Tweets.)
My last tip for Tweet frequency, is try to avoid Tweeting multiple times in a row. When you Tweet more than once in a 30 minute cycle, you will most likely dominate timelines and people will view that as spam.
For more tips on how you can manage Tweeting actively, see my blog about Twitter Scheduling.