What's the difference between "eyewitness" and "witness" in definition? Are they interchangeable?

Can I replace "talking" to "saying"? Do both sound natural?

The expression is to "talk sense," so "say sense" sounds unnatural and awkward in this context. You can use the expression "talk sense," or alternatively, "make sense". Ex: Don't talk nonsense. That's just a rumor. Ex: The meeting went on for hours. No one was talking sense.

Why is the word "about" necessary in this sentence?

Here, "about" is similar to "approximately" or "roughly"- it expresses an estimation. So, out of all the birthday's the kid could have had, this one was probably the worst! Ex: This is about the best day I've ever had. Ex: Wow! This is about the first time I've failed at something. Ex: This is gotta be about the softest blanket I've ever owned.

What's the difference between "social media" and "social network service"?

That's a very good question. Basically, "social media" is a platform for giving information or "content". We use it to refer to what is being uploaded e.g. a blog, video. You can think of it as a one-to-many communication method. Although people can respond and comment, you aren't sending the media specifically to any one person. Whereas "social networking" is a way for communicating with one another, the communication has a two-way nature. Social networks are about interacting and creating conversations. Ex: I consume a lot of social media, like YouTube and Instagram. Ex: My family communicate a lot through social networks like Facebook and Whatsapp.

What's "indeed" mean and when's it used?

"Indeed" is an adverb and means "without any question" or "truly." You can use it the way it is here; to further emphasize a point that's been made or confirmed. Or to emphasis a point you want to make. It does have a slight formal tone to it, but it can still be used in casual conversation. Ex: It is indeed summer now. It's so hot and humid. Ex: A: I think we should buy reusable cups for the party. B: Yes, indeed, I think you're right.

What does “one-hit wonder” mean?

"One-hit wonder" is a derogatory phrase that means that a singer, band, musician. etc. is famously known for only one song but no others after that. It can also be used for other famous content but is typically used for music. For example, the British band "Right Said Fred" is famously known for the song "I'm Too Sexy." However, that's all Right Said Fred is known for. If you were to ask someone what else they are known for, many might not be able to think of another song. Ex: Come On Eileen is a one-hit-wonder. Ex: The band Chumbawamba is known for their one-hit-wonder "Tubthumping."