Found out more details during night:

so its not the operating system hanging. just a few apps seem to use so much cpu, that all the system is slowing down. I left the device charging and turned on during night, with x-terminal top activated.

when trying to activate with the slider on the right, it took several seconds to activate. when trying to activate with the to buton there is the risk to stay to long on it and shutting down the device.

top shows the following (cpu > 3; memory):

user/bin/facebook 7.1 - 12.4; 1.1
user/bin/dbus-daemo 8.3; 3.9
user/bin/feedservice 7; 2.1
(mmicqd) 4; 0.0

system reacting so slow, that messages like "hildon-home does not react anymore, do you want to close it" come up. Uninstall could not be started.

After reboot i uninstalled all of the facebook apps, some could not be uninstalled but were uninstalled from other uninstalls:

Facebook Desktop Widget 0,5.3
Facebook Installer 1.0.-4.1.11+OU.5
Facebook Sharing Plugin 0.5.1 (including 2 Services?)
Facebook Widget and Photo Uploader 0,5.3

Since then the system is running stable for about 2 hours now. But i will try to reinstall later.

Anyway, i will learn how to do reflash and how to backup correctly (i heard the build in backup-solution is only for a few data like pim).

@one1002: Thank you, i will have to read though a thread describing how to backup to external media.

@gabby131: I will reflash, after having learned about it. I suppose my ota-update hat some other problems too (thank you for that hint) and i wand everything to be corrected. hope there are german images available.

@9000: Thank you, i found that mobile-phone-carriers (currently 3) in our country do have different quality concerning theyr services. I did stay with a more expensive carrier which is knows for good support and quality. I also realized with my previous (htc-branded) device, that the carrier usually has to define settings for every supported phone (mine was imported and not supported). as for the n900: it is offered by my carrier and therefore i suppose it is supported too. while discussing with the support they told me that the network-technology is all from ericsson. so there might be minor differences too concerning that. I'm able to switch from 3g to gsm and back using the switching-widget and i always disconnect the mobile-data-connection when i don't use it. this seems to work good. - hey, the lord bless you and your family (and your n900;-) too.