I'm guessing that most web developers have experienced clients who want their website to be the end-all be-all website by creating one just like Facebook's website, or whatever is hot today. The idea is to take users from Facebook by building the world's greatest website for whatever your customer sells and then adding all the things that Facebook does. After all these years, it still amazes me the clients think this way. Do them a huge favor, don't let them! Educate your client on what matters and you will both be happy.
Ask your client, which social network is more popular, Facebook's or McDonald's? They will probably answer "I didn't know that McDonald's offered social networking", and make the point for you. You simply don't go to the hardware store for oranges. Therefore, it is best to concentrate on building a compelling website for whatever it is your client specializes in. Adding features from popular sites won't help, but communicating well and creating real value for visitors will.
However, your client is asking for something here. What they want is for visitors to love coming to their website, find benefit from using it, and ultimately buy the product service or whatever it is that you client is trying to promote. They want their to have all the attributes of that popular website, in order to be successful. If you ask them, they will tell you exactly that. They want to be popular and successful, just like...whatever.
If you've been following my posts, you know that I am a big fan of content management solutions. I think that any developer who is not building this way for their clients is doing them a big disservice. Yes, even a one-page website. And content managers make it much easier to create amazingly capable sites with much less effort than in the past. It is possible, for example, to purchase plug-in functionality for content managers that enable them to emulate social networking sites and search engine sites very well. You could establish a new online dating service, a Yahoo! like directory or something remotely resembling Facebook with a few hundred dollars worth of CMS add-ons. But seriously, who cares?
Don't spend your customer's money poorly because you are too afraid to give them the honest feedback. Stick with the fundamentals here. Value is derived by providing what the visitor is actually looking for. This typically means clear and concise content, accessibility, and focusing on telling the customer's story in a compelling way. Don't use "features" unless they help to do those things.
Be considerate and methodical with your clients and they will be grateful for it. Suggest features that you think will actually help achieve the stated goals and tell them why. If your customer insists that they want to be the next Facebook, Google or Twitter even in the face of your advice, do what I always do. Promise to build them exactly what they ask for even if it goes against all the advice you give. Ultimately, the customer is always right. Just be sure to do your best to inform them and make sure they have the education they need outside of their own observations. They are not webmasters or developers. You are! Be a good one.