What kind of trees are we talking about anyway?
I'd like to address some misconceptions that are in your question before I answer your question directly. You wrote:
If you ask me, one of their goals should be to create a human family
tree. Instead, at the moment every user creates his/her own tree,
without being able to associate one person to multiple trees, and this
information can only be brought to other websites by exporting a
GEDCOM file as far as I know.
FamilySearch has two ways for people to enter family tree data on their site. One is by uploading a GEDCOM to the Genealogies section. A FamilySearch Research Wiki article which describes the various efforts that have taken place over the decadues is here: FamilySearch Genealogies. You can search Genealogies by choosing it from the main navigation bar on the FamilySearch website.
However, FamilySearch does have a "one-world" collaborative tree, namely FamilySearch Family Tree, which can be accessed via the menu item "Family Tree" on the main nav bar on FamilySearch's home page or main search page. An article in the FamilySearch Research Wiki lists some of the resources available to people learning about FamilySearch Family Tree: Training for FamilySearch Family Tree.
So I would suggest that any researcher's first step, before putting information online, would be to understand the site. Know what kind of tree it is, understand what will happen to the information, assess the privacy settings available to the users.
Is the site a collaborative family tree, like:
- FamilySearch Family Tree
or does the site give you space to post a family tree only you (and perhaps people you invite) can control, such as:
- AmericanAncestrees (NEHGS/AmericanAncestors)
- A tree you host yourself
See Amy Johnson Crow's article What Kind of Online Family Tree is Right for You? for an overview of the pros and cons of the different types of websites.
Read the terms and conditions for any site (example: Geneanet terms and conditions) so you understand what will happen to your data before you put it online.
Before you consider syncing your data across online trees, you need to know how each site works. Many a user has "put all of their family history" on FamilySearch's collaborative Family Tree, only to be upset when another user of FamilySearch Family Tree changes it later, erasing all their work. (Prior work remains in the change logs, but not all users are aware of that. They don't know how to look for the changed information or how to trace changes when profiles are merged.)
Whatever method you choose, you should keep a copy of your own work in a location that you control. Work in one location, then transfer the information to the others.
What to Use for Your Main 'Repository' of information
Everyone's workflow is different. The quick answer of how to keep track of your genealogy information is "use what works for you". Here are some things to consider.
Going Old School: Recording information without genealogy software
Some professional genealogists don't use genealogy software on their desktop computers at all. They use ordinary office software, word processors and spreadsheets, a process sometimes called "Write as You Go". It is easier to keep track of your progress, and quicker to write up research reports for clients, if you write up your research as you are doing it, instead of doing a big bunch of research and then trying to write about what you did afterwards. We can do the same, even if the the client is us; we can write for ourselves.
It is possible to combine this approach with having online trees for "cousin bait" or what I like to call "hint bait" or hint harvesting. Your workflow might include keeping a journal of what you've added to your main genealogy files, along with a checklist of which of your online trees have been updated to include your new information. (See the resources list for more.)
Desktop software that syncs with an online tree
Before you attempt to keep your online trees in sync, consider exactly what information you want in your online trees, and what you want to keep in sync across different platforms.
If you are using desktop software such as RootsMagic or Family Tree Maker to send information back and forth between your desktop and Ancestry, be aware that not all of the information that lives on your desktop software is sent to your Ancestry Online Tree. See MacKiev's overview on the About Sync page.
Also be aware that syncing with an Ancestry online tree involves risks. Sending data back and forth includes the risk of files becoming corrupted and data being lost or reduplicated. If you use a package that syncs between the desktop and online, keep your files in order to lessen the risk that you'll lose data (see Resources).
Desktop software that helps you review or discover hints
Many packages have features that allow you to search for information or review online hints, such as Family Historian's Automatic Internet Data Matching which searches FindmyPast and MyHeritage, or Ancestral Quest's TreeTips and Internet Searching. Packages that sync with Ancestry, such as RootsMagic and Family Tree Maker, also allow you to review hints on Ancestry. Stand-alone utilities such as GenSmarts look for gaps in your research and help you find records both on and off-line. If you use any of these, note what you add to your master file so you can transfer the new data across to the online trees you're tracking.
Keeping a research journal
It's important to remember that our lineage-linked software is designed to keep a record of what Janet Hovorka calls our personal conclusion tree. One advantage of using a program like Word or Libre Office, or software like Scrivener as a research journal, is that you can write down your thoughts about your research. Another advantage to a journal is that it provides a place to record information that you haven't reached a conclusion about yet, information that applies to many people (such a locality research), and other data which doesn't fit neatly into standard genealogy software.
Use the workflow that works for you, create a checklist for your process, and follow the same procedure every time for the best results.
- Unpuzzling Your Past by Emily Anne Croom, a basic how-to guide, including photocopy masters for research logs and other forms that you can use to track and analyze your research. A companion Workbook includes 21 revised forms and 21 new forms not included in the original book. These books give the user who chooses not to use lineage-linked software a system to organize their research.
- "Write as you Go" books from Family Locket: Research Like a Pro and Research Like a Pro with DNA by Diana Elder and Nicole Dyer
- Genealogy Research Log (Excel download) and other Genealogy Cheat Sheets from Thomas MacEntee's Genealogy Bargains.
Guides for software packages that sync with online trees:
Feature Lists and overviews
Putting it all together