Perhaps there was a time when you might have known everyone working on questions related to yours. A handful of meetings might have let you know all there was to be known in your field. If you missed a meeting, a journal or two would keep you up to date.
Those times are over. You cannot hope to keep up. Even though you might get away with knowing just one language, if that language is English, there is not time enough in the day to keep up with any but the smallest corner of the academic world. What to do?
You might decide to stick with that small corner of people you know who work on the same problems approached in the same way as you. That way you can all cite each other in mutually fulfilling ways. But we know that creativity comes from reaching out to new fields, from reading broadly as well as deeply. How can you possibly keep up?
If nothing else, teaching keeps you alive because you have to do a certain amount of reading for class. Journal clubs can keep you reading a paper or two a week. If you agree to referee for journals (you should), that also helps keep you up to date in areas close to your own. If you have scientists as friends on Facebook, they will let you know about the more exciting studies. At very broad levels, Futurity lets you see what is exciting out there. You can also get the tables of contents of your favorite journals emailed to you, so you can read the titles and occasionally the abstracts.
I think the most important stuff to read is material that may change your career. There is no simple way to find this except an openness to novelty. In a way it is the opposite of keeping up. You could jettison the idea of keeping up and instead read broadly, only going back to the field you know best when it is time to write.
When you write, you should know what others have done, but you don’t have to have kept up to be good at this. These days you can look forward and backwards with search engines, like the free Google Scholar. You can penetrate even novel webs of citations. By focusing on keeping up only when you need to write, you can cite well and carefully while still saving time for general reading. That way you won’t miss things like the originator of proximate and ultimate approaches to biology. It is Ernst Mayr, by the way, not Niko Tinbergen. I’ll leave the looking up to you!