This post seems to indicate that you want help with crafting the thesis statement rather than an answer that explores the advantages and disadvantages.
The internet has made a lot of things possible. One area that the internet has changed is how personal interactions can be done. Your question correctly identifies that the internet has given people advantages for building and maintaining personal relationships that weren't there pre-internet. Unfortunately, not everything regarding the internet and personal relationships is beneficial. There are some disadvantages to the internet regarding personal relationships.
The first thing to keep in mind about a thesis statement is that it is going to make an argument. You could use an overly simplistic thesis statement that says, "There are advantages and disadvantages of the internet on personal relationships." In my opinion, this is not a strong thesis statement, and this question asks for a strong thesis statement. The reason it isn't strong is that it doesn't show that you have taken a side regarding whether or not the benefits outweigh the negatives. The statement isn't pushing your reader to change their attitude about anything. Your reader is likely to already know that there are pros and cons to using the internet for personal relationships.
Making the thesis stronger isn't that difficult. It requires you to tweak the initial, weaker statement into a dependent clause followed by an independent clause that contains your main counterargument to the first half of the phrase. The thesis might look something like the following statement:
Although there are serious disadvantages to using the internet for personal relationships, the various advantages of the internet for those same relationships far outweigh the negatives.
Admittedly, this thesis statement is a bit vague, because it doesn't mention any specific advantages or disadvantages; however, this initial statement can be expanded upon as you fine-tune your argument.