What abstract things can you 吸收？
Basically anything that can be absorbed as nutrients, literally or figuratively. Yours are just a few examples, we can have more!
A plant may absorb the essence of the nature, or 吸收日月精华, and become a genie.
A political party may want to attract and invite the best people to join. Mao Zedong wanted more learned people in CCP, so he wrote 《大量吸收知识分子》. Now CCP is still trying to 吸收入党积极分子 in colleges, companies, and danweis.
Search 吸收 for book titles and you get plenty:

有吸收力的心灵 A spongy heart?
清末民初民間儒教對主流儒學的吸收與轉化 Absorption of mainstream Confucianism
早期天台学对唯识古学的吸收与抉择 Absorption of certain academic topic
三十天吸收三十年專業能力 Absorption of certain abilities
人力资源吸收与选拔 Absorption of human resources
技术吸收型对外贸易战略研究 Absorption of technology
中国吸收外资30年 Absorption of capital investment
知识的吸收与创造 Absorption of knowledge

A company may completely buy off another and it's called 吸收合并, in which case the company being bought is likely to be dissolved and will no longer function as an entity.
There is a criminal offense called &quot;非法吸收公众存款罪&quot;. You illegally collect people's money and do your secret stuff.

I don't think I'm finished, but let's just stop for a while. Your two examples are not colloquial because, well, you wouldn't say these in English, would you:

I come to Taiwan to absorb knowledge.
I joined Facebook as a summer intern to absorb their knowledge.

Same reason, if it's personal, and if it's casual talk, the word absorb or 吸收 will sound too fancy. You usually say to learn or to study in such occasions.
Contrary to @Alex's opinion on 思想. 思想 can also be 吸收-ed. 吸收 is an active activity1. You would need to provide the specific topic for 吸收 though, as in 吸收儒家思想. On the other hand, I can't think of any example in which 信念 is 吸收-ed.
These are just my opinions. Have I mentioned that opinions can also be absorbed? 也许吸收了我和别人的意见以后，你会形成一个更完整的看法。
1. Sorry, running out of vocabs.