It seems that whenever I choose optimized for clicks or optimized for impression (default setting), it would take another day or more for the impressions to start trickling in. Moreover, not all ads receive the same number of impressions. One ad would get about 80-90% of the total impressions I get. Does anyone else have the same issue?
There is quite a lack of information here...
What bidding mode? Budget?
What are you actually bidding?
What kind of ads?
What country?
How are your ad sets structured?
It looks like you're lumping ads together into the same ad set - don't - this is going to cause impressions to eventually funnel to one ad more than the others as a result of Facebook's optimisation process.
Without knowing anything specific about your demographic and bids and budget, we can only speculate about why you might be getting a trickle of impressions.
I'm doing right-hand-side ad, optimized for clicks so there's no bid limit, the budget limit is $5/day; I'm bidding on male 18-23 in NY USA with a specific interest. The demographic for this is 114,000. I have one ad set and 2 ads in this ad set.
Actually my impression doesn't trickle. There's no impression at all after more than 24 hours. Kinda strange 
Uhhh, if you bid CPC you have to put in a bid.
Choosing auto optimise for clicks means you are using oCPM.
If you are not getting impressions with oCPM, it's either because you have a very small audience, or more likely, your bid is too low.
Go to the power editor, find your ad, and set a weight to the oCPM box for clicks. It is effectively an average CPM bid. If $5 CPM doesn't give you traffic, try $6. $7, $8 etc.
Thanks Zeno, I put in a high bid for CPC and got impressions instantly. However I notice that the quality for right hand side ad is just horrible. Out of 14 clicks that I got so far, there was no landing page clickthrough at all whereas with newsfeed ad, I get pretty good landing page CTR and even some conversion.
The RHS and News Feed ads might as well be classed as belonging to entirely different platforms.
There is the FB news feed.
There is the FB RHS.
The RHS is pretty garbage now and you'll get very low CPMs but low click through rates. Landing page CTR isn't something I'd say would tie into the ad being RHS, might just be a batch of crappy clicks or the ad is simply not engaging people in the same way as the news feed.
No you can still make money through RHS ads - as with every traffic source you just have to test test test to find what works and what doesn't.
For the news feed I'd say anything above 4% is good, undoubtedly some people are getting 10+% in some campaigns/demos.
Thanks for the really helpful info Zeno. It seems that I suck with choosing ads. My newsfeed can't get above 0.3%. What kind of pictures do you think can get that high of a CTR (4%-8%)? Like girls with some skin showing or people looking right at you or some weird pictures?
It depends on a combination of your ad images/copy, targeting, and the angle you use.
No image alone is going to be a constant 5+% CTR bombshell.
Weird pictures with viral potential might get you high CTRs but how does that help you sell a product/service?
Direct response marketing will always require a balancing act of attention, interest/desire >> action and image CTR primarily influences the first of these.